Two things I'm wondering about:
#1. If you overclock the CPU will you effectively be downclocking the GPU ?
This might seem a bit odd, but as far as I can tell it might be the case. As we know all parts of the processor share the same baseclock, so overclocking the CPU will have to be done by increasing the multiplier, which won't affect the GPU. However, if you overclock the CPU you will be operating above the TDP most of the time, which means the GPU turbo won't be able to activate, thus effectively slowing down your GPU.
#2. Can you overclock in steps smaller than 100MHz ?
The GPU shares the 100MHz baseclock with the CPU, yet it uses speeds that aren't divisible by 100MHz, which means it appears to have a multiplier that isn't a whole number. If the same holds true for the CPU it might be possible to overclock in steps smaller than 100MHz, since you might be able to increase the multiplier in steps of 0.5 or even 0.1
For all we know you can overclock the GPU same as the CPU, separate multiplier, separate Vcc.