Here are some lines from volume 41 of the manga (Viz copies) about Rou Dai Kaioshin's ki unlocking thingy.
Gohan: Shouldn't we be done by now?!
Rou Dai Kaioshin: Huh? What time is it? Hm...if it's not done yet...you musta had plenty o' latent power in ya!
Goku: "Latent power" is hidden power he had inside...right?
Kaioshin: I...I think so...
Goku: How can that be...? ...How much power did Gohan have hiding...?
It seems to imply that all of the power being released is Gohan's. The power he'd be tapping into if he could transform an extra time or two, since Rou Dai Kaioshin later says "transformations aren't everything. That super whatever of yours was just show biz."
Edit: Onikage's theory makes a lot of sense from many angles. I think when Goku and Gohan trained to make SSJ their norm (FPSSJ) they greatly reduced the ki wasted to maintain the form (thus adding that wasted ki to their reserves and increasing their stamina), reduced the restless feeling and, pesonally, I think they greatly reduced the stress it puts on the body too. Like gravity training, eventually your body gets used to the strain and it doesn't count as adequate training any more. This is probably why Goku and Gohan reached a peak and didn't think it would be worth training in the RoSaT for another day.
But also it's about training time. Someone might think, "Well, if they train in their base forms and power up, SSJ will automatically multiply that. So why even bother training as an SSJ any more?" The answer (which may be what Onikage was saying, but I only just really got it) might be that, let's say you're in your base form and you train, gain a little strength, train, gain a little strength, etc. Eventually you outgrow the current level of training and have to move on to something harder.
But if you're SSJ and every little gain of strength is multiplied, you would find yourself outgrowing whatever training you're doing faster, maybe 50 times faster (maybe not, not sure if the 50x power level multiplier equates to training time directly, but you get the idea). You move on to greater training, and outgrow that quicker than you would in base form too. In terms of training time, SSJ forms seem logical in that sense. Although I do think they eventually reach a peak and it becomes harder to increase their strength without moving on to the next form up.
Does that make any sense? It's late and I'm tired so I'm not sure if I'm thinking logically.
