Well, depends on the 260 version, 260 sli performs inbetween a GTX 470 and GTX 480, and a GTX 560 isn't much more powerful than a GTX 470, in terms of raw-performance the GTX 260 SLI probably is a little more powerful.
Honestly, I don't even think a GTX 580 is a powerful enough upgrade from my GTX 260s to make it "worthwhile" I'll be waiting for the GTX 595.
Did you even read his post?
What exactly does any of your post have to do with anything in this topic?
The question is "Why does he get a lower Vantage score with a physics card installed then he does without a physics card installed"?