What you are missing is that core and shader clocks are related even in unlinked mode.
You can't set the core over certain frequencies if you don't clock the shaders higher aswell: when the shaders are at 1458, max core frequency will be 712; at 1512, the core will top at 748 etc.
That is caused by the different core/shader ration GTX series have, which is different than 8800's.
I've heard there are people who are trying to change that ratio through the bios but don't know the details.

About software voltage adjustments, considering the same Volterra chip is being used I really hope W1zzard can sort it out like he did with ATItool for 2900s.
Looking forward for the ATITool substitute he's been talking about