I'll try to explain this.
IMGTech, makers of SGX, is like ARM - they don't sell mirochips, they sell designs. This means they license you the rights to take their design (distributed in some low-level abstract hardware description, sort of like an executable file - pure data, but a black box you can't study or modify) and incorporate it into your design of some chip you're making.
When they say that SGX can clock at 200MHz it means that it's possible, using some readily available process technology, to implement the chip such that it'll reliably operate at 200MHz. But if you choose another technology you could be limited to something slower (likewise, if you reimplement it in a technology 5 years down the road you could possibly clock it much higher). Also, the other components of the greater design can be limiting the clock speed, especially if the clock is shared with other domains. Or perhaps the memory subsystem won't work at higher clocks.
In TI's case, OMAP35x is using a 65nm process and probably one optimized for low power consumption instead of speed. Process decisions are going to be determined by price, stability, availability, power consumption, and how much the rest of your design can be made to work in it. Since the original OMAP35s were released TI has come out with TI35x on a 45nm process, which is allegedly capable of clocking the SGX at 166MHz.
Incidentally, you may be wondering why the best commodity processes can only get SGX 530 to 200MHz while desktop GPUs can reach much higher clocks. If you want I can explain that, but I won't bother unless someone really is wondering.
