I did have one kind of sleep issue at 2666mhz memory settings.. Not sure if it's related.
In fact, I set the machine to never sleep and just shut the monitor off because I had some big downloads to do.
The screen would not wake!
I set darkwake back to =8 and haven't had an issue so far. I will leave that on. It doesn't seem to interfere with anything, although I didn't connect a meter to see if there was any odd power draw.
Someone else mentioned that their machine still drew 50 watts or something in sleep mode, but I don't remember the specifics.