Tesselation sucked on pre 7000 AMD because of something to do with little caches internal in the GPU I believe. They werent big enough.
It's fixed in 7000 series, which is why games with heavy tesselation often see their biggest gains on that card.
The idea is Crysis 2 tessellation (and all tessellation) is borked on 6000/5000, but it isn't on 7000. For example
you're talking about a 56% speedup 7970>6970 there, where the average is more like 40%. It's the same in any tess heavy game.
Thats one reason I'm holding out for a (pitcairn) 7000 series rather than jumping on a 6870 or something, much more forward looking.