We've been running the cortex A8 to higher freq than 600 . It will not harm your phone to run it at600 but battery life will be shorten for sure.
Anyway the sgx in completely integrated into omap3. There is 4 processors in the 3430: CA8, C64 (dsp), the SGX + the image processor. The ARM has it's own frequency, that is differentiated from ddr/core one.
By the way, omap3430 is 45nm versus 65nm for snapdragon, which is where come a part of its consumption advantage. The second is the slower frequency, which may not appear as an advantage in front of phone/cpu addicts. But actually it's a significant advantage to spare battery time and achieve sufficient performance.