I think in terms of AA it's just that they've got tons of raster power to spare because AMD gave them 32 ROPs (half this amount managed to keep up with the 32 ROPs in the GTX2x0 series in terms of AA performance) in anticipation of them powering stupidly high resolutions thanks to Eyefinity. It's not that they're CPU limited, it's more they're everything that isn't raster operations limited at any resolution lower than insanexstupid.
I suppose yeah, that makes sense. Increasing the res doesn't reduce performance as much as some people expect really.
It's the AA at the increased res that eats performance.