Good question, I've no idea to be honest. I was hoping some of the experts here would share some thoughts on the matter.
Been doing some searches on the matter but can't find that much (6 pin is 150 and 8 is 300w, I got it mixed up a bit)
However this post is rather interesting:
The Ultra X3 1000w won't work with a (single) gtx 280 for several people. due to the 6+2 pin. Too much volt drop. It does however work for some even with two g280's in sli.