About the copy Vegeta discussion, I think the point in that arc was that "purple" Vegeta was an exact copy which implies that it has exactly the same power, same techniques and the same mind; so no, it doesn't amplify power.
[spoiler]
Context: Copy-Vegeta appears.
02:53 | Vegeta: "Who the hell do you think you are?!"
02:56 | Vegeta: "Transforming into an exact copy of me..."
02:58 | Copy Vegeta: "I'm not a copy."
03:00 | Copy Vegeta: "I've taken all of your power. Now, I am Vegeta!"
03:06 | Copy Vegeta: "You're nothing more than an empty husk!"
03:11 | Vegeta: "What the hell kind of nonsense are you spouting?"
03:13 | Vegeta: "You're nothing but an impostor! Drop dead!"
03:22 | Vegeta: "What?! I can't fire..."
03:25 | Copy Vegeta: "Is this what you were trying to do? Gallic Gun!"
Significance: Shows that the Superhuman Water is capable of stealing other's power and taking on their appearance. Also shows that the Superhuman Water copies more than just raw power; Copy-Vegeta knows all of Vegeta's techniques.
---
Context: Copy-Vegeta is ordered to absorb the others.
03:57 | Copy Gryll: "Now, absorb the others' power as well."
04:01 | Copy Gryll: "How about that kid?"
04:05 | Copy Vegeta: "Trunks..."
Significance: Copy-Vegeta recognizes Trunks and hesitates to kill him, showing that the Superhuman Water can also copy one's mind, as Potage later explains.
---
Context: Vegeta and the others escape into a cave.
05:43 | Potage: "What attacked us just now was the Superhuman Water."
05:45 | Jaco: "What? Superhuman Water?!"
05:49 | Vegeta: "You know of it?"
06:08 | Jaco: "I believe it was a miraculous water,"
06:09 | Jaco: "that will increase your power several hundred times over if you drink it..."
06:13 | Jaco: "I didn't think it actually existed."
06:17 | Potage: "This "miraculous water" doesn't exist."
06:24 | Potage: "The "Superhuman Water" is a name attributed by people from other planets, all on their own."
06:31 | Potage: "Its true name is "Comméson"."
06:36 | Potage: "It's a weapon that we, the people of Pot-au-feu, developed to defend ourselves."
Significance: Reveals the Superhuman Water's origin, its original name, and shows that Jaco was aware of its reputation.
The name Comméson (コメソン) presumably comes from the French word consommé (コンソメ), which is a type of soup.
One translation mistakenly claimed that the Superhuman Water boosts one's power "one hundred times", rather than "several hundred times" (何百倍).
---
Context: Potage explains the Superhuman Water's backstory.
06:47 | Potage: "Long ago, our people, who had no strength of their own, discovered a liquid that absorbed the strength of others, leaving their victim powerless. They used this liquid to defend the planet from invaders. The Comméson's absorption powers were unlimited. It went on absorbing the power of our enemies, then disguised itself as clones of them. But the Comméson had absorbed too much evil power from invaders, and one day, it grew to bear an evil will of its own. It didn't just flawlessly clone others' personality and abilities, it started doing this for one single purpose: In short, it became a lifeform that would only absorb and absorb without end, purely in the pursuit of power. Because it appeared identical to the absorbed victims, only powered up, rumours that it was a miraculous water lured even more invaders here. But the real legend is what's yet to come... If the Comméson's power had continued to increase, then never mind Planet Pot-au-feu... The entire universe might be destroyed! With that in mind, we, the people of Pot-au-feu, sacrificed ourselves to seal the Comméson away. And the sole survivor, which would be me, has spent the last one hundred years plus defending its seal!"
Significance: Reveals that Potage's people had "no strength of their own", and gave their lives to seal away the Superhuman Water, leaving Potage as the only survivor.
Explains why after absorbing someone's power, the Superhuman Water takes on their appearance, explaining copy-Vegeta and copy-Gryll.
Reveals that the Superhuman Water has its evil nature due to all of the evil it's absorbed in the past.
According to Potage, the entire universe may be destroyed should the Superhuman Water be allowed to run wild.
According to Potage, the Superhuman Water can absorb anyone without limitation.
---
Context: In the cave.
10:07 | Vegeta: "That fake has both my appearance and my abilities. In other words, it's practically the same as if I myself had been taken over by the Superhuman Water. But back then, he hesitated for a second to absorb Trunks. Why?! He should be nothing more than Superhuman Water!"
10:28 | Potage: "That is the result of our craftsmanship."
10:32 | Potage: "It has faithfully copied your mind as well!"
10:38 | Vegeta: "I see. I wouldn't go so far as to absorb other people's power to become stronger, after all."
10:44 | Potage: "But his heart is under the Superhuman Water's control."
10:50 | Vegeta: "That thing is me, but he's letting himself be the slave of some monster?!"
Significance: Confirms that the Superhuman Water copied Vegeta's mind in addition to his power. Despite this, the Superhuman Water is still in control. Well, supposedly. As we see later, Vegeta's personality allows Copy-Vegeta to act independently.
---
Context: Still in the cave.
10:58 | Vegeta: "What happens to those who have their power stolen by the Superhuman Water?"
11:09 | Vegeta: "They die, right? Even me..."
11:13 | Potage: "Once your body turns transparent, you'll only have three to five minutes left."
11:17 | Potage: "I'm guessing that they're already..."
11:28 | Jaco: "This is not good! I don't care about Gryll, but if Vegeta dies..."
11:31 | Jaco: "Bulma will definitely blame me for it!"
Significance: Reveals that Gryll and his men are already dead, and that Vegeta will soon follow.
---
Context: Still in the cave. Still.
11:33 | Jaco: "Is there no way to save him?!"
11:38 | Potage: "The only way... The only way is to defeat the Superhuman Water!"
11:42 | Goten: "Defeat him?"
11:43 | Jaco: "The fake Vegeta?"
11:46 | Vegeta: "So it's the same as defeating me."
Significance: Reveals that defeating the Superhuman Water will save Vegeta.
In case it weren't obvious already, we're told that it's the same as defeating the real Vegeta.
---
Context: Outside of the cave. Goten and Trunks fuse in preparation for battle.
14:01 | Gotenks: "Ta-da! Super Gotenks 3 is here!"
Significance: While not in the manga, "Super Gotenks 3" is how the narrator refers to Super Saiyan 3 Gotenks in Dragon Ball Z.
---
Context: Gotenks tries an assortment of techniques.
13:21 | Gotenks: "Miracle Punch!"
13:25 | Gotenks: "Miracle Kick!"
14:27 | Gotenks: "Great Special Rolling Kick!"
14:39 | Gotenks: "Super Special Crash Hammer!"
14:59 | Gotenks: "Finish Special... Flash Strong... Bomber!"
Significance: None. Gotenks prefers cool, flashy-sounding technique names, hence why they're all in English.
---
Context: Sensing ki in battle, Goku appears.
18:44 | Goten: "Dad, be careful! He can absorb power!"
18:48 | Copy Vegeta: "I won't do that! This is a serious duel to the death."
18:56 | Copy Vegeta: "Kakarrot! I will defeat you!"
19:00 | Copy Gryll: "What are you saying?! Absorb his power!"
19:04 | Copy Vegeta: "I will show no mercy to anyone who interferes! No matter who they are!"
19:08 | Copy Gryll: "You refuse to obey my orders?! In that case..."
19:19 | Potage: "The Superhuman Water is trying to force the clone to obey!"
19:25 | Vegeta: "You dare give me orders?!"
19:27 | Copy Vegeta: "I will not be ordered around by anyone!"
19:34 | Copy Gryll: "W-What?!"
19:35 | Copy Vegeta: "Get lost, small-fry!"
19:54 | Potage: "He shook off the Superhuman Water's control!"
19:58 | Copy Vegeta: "Now the nuisance is gone. Let's get started!"
Significance: After the superhuman water copies Vegeta, Copy-Vegeta rebels and kills Copy-Gryll so that he can fight Goku uninhibited.
[/spoiler]
In my opinion, Roshi isn't there because of his raw power, it's merely for his abilities and expertise but... at the same time, if SSj3 Gotenks is a lot weaker than base Goku/Vegeta and Roshi is "apparently" a challenge for Goku, therefore Roshi is stronger than Gotenks (as SSj3!). Just the sheer thought of it is absurd. I can only cope thinking that Goku was hooolding back, as usual, and Muten Roshi was powered up by the whichcraft spell.