Well the numbers are double on the 535, so theoretically it should be able to push out double the polygons. Then again there isn't very accurate info on those mobile gpu's so it's hard to say what kind of performance increase it would actually mean.

And of course the CPU also affects the performance and it is still the same speed as in N900.

And I wouldn't say that the info on 3GS's gpu being the 535 model as 100% sure. The SoC seems to be something Samsung has specifically made for Apple, so hard to be sure what's inside it.