Wow very impressive, i really need to get one for my 955, i hit 62 under full load.
No speed drop from lowering the vcore, your speed will only be affected if you modify the base clock speed or the CPU multiplier, adjusting voltages wont change the speed or performance at all, just dont lower it too far, there is a point where the voltage is too low for it to run stably, at which point just bump it back up another notch or two.