I kinda figured that's the safe bet but if you read Boozer's post he mentions his second PSU powering the 3 powered risers but only 2 cards with regular 6 pin or 8 pin connectors meaning that the card attached to the 3rd powered riser is getting power from 2 PSU's. Maybe I read that wrong...

I'll keep digging as I really would like a definitive answer about this. I know that using different models etc. would probably be an issue but if/when I build a mining rig it would be using the same model GPU's and PSU's which means their variance is whatever the manufacturing process allows and hopefully everything would be similar enough that it wouldn't be an issue, even a small one that takes a year to actually cause a real problem.