I agree with Zahabi, mainly because he knows GSP better than eveyone here on this board.
And secondly, because GSP is the type of person who is bulking up by constantly adding muscle mass to his frame, not just any kind of mass.
Although muscle mass makes you stronger/gives you more stregth, this comes at a price:
1. you become stiffer, more rigid - and that affects you movements in the ring, it affects the way you throw punches and the way you shoot for the takedowns.
Basically: muscle strength kills speed/explossiveness.
2. more muscle mass needs more oxygen. Basically, carrying all that extra muscle mass on your frame while fighting at the same time ... or doing any kind of high intensity physical exercise, will force your body to work harder, by consuming more oxygen, in order to perform at a high level.
That means, GSP would have to imporve his cardio/conditioning even more.
And he already is the best conditioned athlete at WW in the UFC at least. Going beyond this would be much harder.
With that being said, i believe GSP at this weight ~192 lbs, would be a relatively small MW, but still he would beat most of the fighters at 185, thanks to his superior technique and skill set.
But i believe he would have problems against the lieks of Sonnen and Okami, guys who are bigger than him, 205+ lbs on fight night, and have a style that could shut down many of GSP's advantages.
I believe GSP stays at WW for at least 2 more years.
Still some decent fights for him here...