Wow. Where was all this C states discussion when I was piddling with my P8Z68V-Pro (gen1) and i5-2500k about a year and a half ago?! I couldn't find anything at the time to help understand the C states.
IIRC, I left the C states enabled and run a negative offset (haven't messed with the settings in a loooong time). I also haven't upgraded the BIOS past 1101 either and run a very mild OC of 4.2. I have pretty low idle Vcore, if Open HW Monitor is showing accurate numbers. I did a lot of piddling on it at the time and there was a reason for my settings... which I've long since forgotten! I don't think my chip is in the top tier as I had trouble pushing it past a 4.5x OC.
Edit: I noted after posting that my sig shows a 4.4x OC. I changed to 4.2x a long time ago and haven't bothered to update the sig. Just in case anyone notes the discrepancy.Edited by speedlever - 2/10/13 at 4:09am