Hmmm... Bottom line - in my case powered risers and an extra power supply made no significant difference other than lowering the temperature of the card that's outside the case  (16 degrees cooler, tested on one motherboard so far) .

Powertune affects my hashrate a tiny little bit, but i'd rather undervolt (mine is -20, I'll test power consumption when I get a chance). I'll play with it some more, now that I have all this stuff, but so far no positive outcome.