This comes from an old article, but would seem to add some backup to the idea its down to a system bottleneck:



http://www.tomshardware.com/reviews/...is,1572-8.html

if the 275gtx and 4890 are both scaling like their older siblings, in pcie limited situations the ati card should handle it much better, so any platform advantage in bandwidth/latency will not show up on the ati card. but, would show prominently on the nvidia card.

easy enough to test also, see if the cards scale nicely if the # of slots is reduced. if the pattern persists, or becomes more pronounced - nail in coffin. if the change has little effect, its not pcie limitations