Originally posted by dadudemon
Also, I would note that in the anime, Goku, in SSJ3 form, was able to fight for a bit with a version of Buu that Gohan could not compete with. That was not the case in the manga. It is possible that this idea, from the anime, was carried over into this movie. This would be a retconn on Toriyama's part...and it would seem he wants to keep the anime stuff consistant.
I argued the same thing before, regarding SS3 Goku's performance against Buu. I doubt Goku was explicitly stronger than Gohan though. Mystic Gohan is simply Gohan's true potential harnessed in his norman form, giving him power on the same scale as a SS3.
It could be that Goku applied the same energy-conserving training methodology on the SS3 form during that time-period that he did on SS1 for Cell's tournament.