I'm having the same problem. The multiplier/ frequency drops but the voltage always stays at the highest p-state.

But there is a way to enter into dsdt, the p-states and c-states available for your cpu. Still researching this but I am going to give it a try.

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GA EP35-DS3P - Intel Q9550 - 2 X 2GB Dominator 1066Mhz, EVGA 8800GTS 512MB
2x320Gb SATA in Software Raid 0, 1x1.5TB for Time machine and storage (Geekbench 6293)
10.6 Retail 64bit Using DSDT for audio-889a, video-8800gts, ethernet w/TM fix, USB fix, and SATA fix.
MacBook 2,1 - 10.6 Retail
MacBook 3,1 - 10.6 Retail