As you say, undervolting can cause stability issues, but I cannot really see how dropping the CPU frequency should affect the stability of the CPU itself. Undervolting would have much more (squared) effect per unit voltage change but I was just thinking since the drop from say x32 to x8 multiplier is quite a drop in frequency. Since Power = C * (V)^2 *f, (C= capacitance = 'fixed load', V= voltage, f = frequency), dropping either f or V should help with battery life. I thought that dropping the multipliers on the CPU would drop the frequency and therefore drop the power consumption.
I have also heard of Nvidia Optimus Technology which I don't think is available on the 965M in the G751JL. I guess we would need some way to turn off the NVIDIA GPU (965m) and use the CPU Integrated Graphics to save power (switchable graphics). Good point about the GPU burning most power. Undervolting the CPU will probably affect your stability and won't get you any significant gains.
Major part of the energy consumtion is due to the GPU being on at all times. You won't notice any difference, because you can't save anything with dropping the frequency, you will have to undervolt as well.
