Thanks Greygoose!
I just reached 50+ hours uptime on 6.4.0-rc7a with C-states enabled. Looks like Lime-Tech may have solved the stability issue, good job guys!
With C-states enabled, Idle wattage has dropped 10+ watts. My UPS only reports in 10.5w increments (which is 1% of the 1050w power rating), so actual savings are likely somewhere between 10.5w-21 watts. From earlier testing with a more accurate Kill-A-Watt, the actual delta between C-states enabled & disabled was between 12w-18w.
Idle temps have dropped 2-3 degrees C on both CPU (41C) and System (36C). Not as much as I had hoped, but I think my expectations were off. I did a lot of initial testing with the case cover off, and temps have unsurprisingly increased simply from closing the case, as case fans are on lowest speed (three 120mm fans, 1000 RPM @ 35% PWM), and they have to suck air past the HD's, so very little airflow at idle. The CPU fan speed profile is set to 'Standard' in the BIOS.
At max case fan speeds (2750 RPM), idle CPU temp easily drops to 35C and System to 30C, but the higher fan speeds consume an extra 10+ watts and make lots of noise.
As a compromise, I just changed my minimum case fan speed to 1400 RPM @ 50% PWM, which is much more quiet and energy efficient than full blast, but still improves my idle temps a couple degrees over the slowest fan speeds: 39C CPU, 34C System. I'll probably change the CPU fan profile from Standard to Performance in the BIOS to see if that drops the 5C delta over ambient a bit, but other than that I think I'm done.
I'm happy to have idle temps back in the 30's, at reasonable fan speeds/noise, and with idle watts back to a more reasonable level.
Paul