Well, GT5 Prologue didn't have any of GT5's issues - it runs much better than GT5, doesn't have screen-tearing, jaggy smoke effects etc. and all that while having better graphics (no standard cars and tracks). That means that GT6 doesn't have to sacrifice anything, it just a matter of proper programming, which apparently was screwed during GT5 development.
It does push PS3 to it limits in some aspects - it runs at ~60FPS, which already eats half of the power resources that other PS3 games you've mentioned use for better graphics. Secondly, the best current games like Uncharted, Killzone, Crysis, Battlefield pull out something between 1-1.5MLN polygons, while GT5 uses 0.5MLN polys PER CAR. Now multiply that number by 16 and add the whole track to it - NO game comes close to GT5 as far as poly count goes.