you should be able to use all c states matt and speedstep,ive never had trouble with them
cant see x79 being any different,you might need few clicks more offset than a fixed voltage tho
Auto i think disables them, or at least some of them.
Efficiency score isn't really a realistic performance gauge. You can wack down the refresh interval on some sticks to silly low and it will give you a huge efficiency but the performance will be slower.
Also I found c8-9-9 to be slower than c8-10-10 at 2133
One of mine died at 1.7v but don't let that stop you![]()