I'd like to present a pseudo-theory on the whole "SS is 10/50 times!" thing. Sort of a theory... bear with me. It's a guesstimate based on... stuff.
(This is out of universe stuff, right, because it deals with what Toriyama intended as a writer, and not internal justifications as if the story were real? Right forum?)
When people heard this quote; "At the time, it was made out that he was fifty times as strong when he became Super Saiyan, but that's a little extravagant. As far as my feelings as an author go, I think I drew it with the sense of it being a change of about ten times what he had been up until then", the reaction by and large was that this didn't make sense! If Toriyama thinks that 50 times is too excessive, then how can 10x Kaioken 10 or 20, which would yield multipliers of 100 and 200 times base respectively, not be even more "extravagant"? Is this a case of Toriyama just being forgetful?
Maybe it's the most likely explanation, but I want to raise another possibility. When we think of Super Saiyan now, we think of the golden warrior Goku became at that exact time after Freeza killed Kuririn. However, artistically speaking, from an out of universe perspective, this may not have been what Toriyama had intended all along. At a certain point, he had the idea for a legend of the Saiyans, and then he gradually developed more of the idea as we want along. We know that the idea to have the Super Saiyan be a form which golden hair and light eyes was created at some point after the basic concept was; perhaps for inking reasons, as much as drama, Toriyama hints.
Looking at it from a forwards perspective, rather than looking backwards with what we know now, we might say that at a certain point, when he was actually thinking in terms of multiplying battle power, Toriyama did intend for Goku to multiply by ten times to be a Super Saiyan. However, with the amount of delays in getting to that point, and the way the story was largely made up as he went along, Toriyama changed tack as the power level progression required, and the Super Saiyan became the explicit form which is greater than Kaioken, as opposed to the simple state of being, or level of power that Vegeta kept implying before then. *phew* That was a long run on sentence...
Is there any evidence for this? No. Not really. I can only present it as a possibility, but I can explain my gut feeling on this. It's clear that Super Saiyan didn't appear fully formed in one go, and it's also clear that Vegeta kept up a belief that a Super Saiyan was simply a stratospheric legendary level of power rather than a form (Only the anime shows a golden ape. At what time was that?). From an in-universe perspective we can say that Vegeta just didn't know. Even an out of universe perspective allows us to say that Toriyama knew all along exactly what SS was, and at what level of power it would appear, and was just using misdirection for the sake of drama. I'm skeptical of this because I know of how Toriyama wrote the series; he had some overviews of things generally, but largely he claims to have made up the story chapter by chapter. Toriyama also hints that there was a period when the SS concept wasn't complete.
There's something else as well. In the ending pages of Chapter 270, we read this:
GOKU: "Now I can even take Kaioken 10!"
*next page*
NARRATOR: "Goku doesn't know... that he now has the potential to become a Super Saiayan."
Well now... Did Toriyama originally intend for the Super Saiyan to be Goku at Kaioken x10? It fits with thinking of the battle powers in terms of a 10 times increase when he was drawing it, and all the time Vegeta was hyping Goku (and himself!) up as having become Super Saiyans, the KKx10 had not yet been shown off. Could Vegeta's hype have instead been intended to eventually resolve with Kakarot unveiling his KKx10, defeating Freeza, and attaining the status of a Super Saiyan?
We'll never know. The power levels check out though. Goku at 90,000 using the KKx10 reaches a power level of 900,000; enough to beat Freeza's 530,000!
However, Toriyama has Freeza transform many times, and moves away from giving measurements of their battle power (and probably keeping track of it!), and many other Zenkai powerups occur based on making up the story as he went along to fit the battles. By the time Goku actually fights Freeza, Freeza is in his final form, and Goku has gained a gigantic power up. The battle progresses more, until KKx10 is shown to have been used already, and is still not enough to beat Freeza. We don't know when, but at some point here, the golden haired warrior may have been invented, and after many many chapters of fighting it is finally revealed, and by this point the KK logic makes a 10 times multiplier iffy... but Toriyama wasn't keeping track of battle powers at this point!
Is it possible? We'll never know... but it's something to think about if we think about how Toriyama makes his stories, and the 10x move which was hyped just a page from the second mention of SS ever.
tl;dr - Toriyama was thinking of the Super Saiyan as a level of legendary power, and post-gravity training Goku at 10x Kaio-Ken was supposed to achieve it. However, giving the piece by piece way he makes his stories, the battles he wrote led more naturally to events leading away from what he originally intended. By the time, he finally reveals SS and develops it fully into a "form", he'd stopped thinking about power levels, so outside sources calculating what eventually happened based on earlier power levels was a little "extravagant" compared to the multiplier he'd originally intended for SS when he was actually focused on battle powers. Or so says I, with no evidence!
