Whether I agree or disagree, I'm not sure. I do find the idea plausible, however.
When Goku exited the Room of Spirit and Time, he said, "But pointlessly toughening up my body more than this is just torture, not training. But I won’t object if you guys enter that room again. It looks like you still have room left to toughen up more." Some have argued that he just means that the RoSaT training itself is now relatively ineffective due to his vast improvements, but to me the wording indicates that he's only speaking about how much he (and Trunks/Vegeta) can improve in general. Plus, calling further training "torture" certainly sounds like he can still exert himself, he just won't see any benefits if he does so.
For what it's worth, here's what the Daizenshuu has to say:
Goku trained in the Room of Spirit and Time before his fight with Cell. He draws out the power of the Super Saiyan to its limits, and the aura that surrounds him is different.
http://web.archive.org/web/20120308230017/http://www.kanzentai.com/trans-daiz02.php?m=04&id=grow_up#link
"...draws out the power...to its limits..." sounds like he can't improve anymore. The Daizenshuu term for the Mastered SSJ, "Full Power Super Saiyan," also seems to hint at this – it's the literal full power that Goku can ever achieve in that form.
Some things do seem to go against it though. For example, Vegeta saying that if all the Saiyans are restricted to base, then there's no difference (before the tournament in the Boo arc). Perhaps one can assume that base increases as SSJ2 increases, whereas SSJ stays the same. Very strange scenario but nothing explicitly goes against it.
Goku's SSJ increasing in the Boo arc also wouldn't be the first time someone surpassed a supposed limit (if I recall, the holy water on Karin's tower was supposed to unlock all of Goku's hidden power, though perhaps there's a difference between hidden power and the potential for strength increases, I dunno).
