SP is basically completely GPU bound. I dunno how AMD manages it so often but they couldn't have picked a worse game to showcase Mantle with. Obviously they didn't know this would be the case, but still.
The scaling is near perfectly in line with what we know about each card. 290X ~30% faster than the 280X which is ~20% faster than the 270X etc. Nvidia appears to have squeezed a few more percent out of their DX drivers and that's why the Ti has a 13% lead instead of say 6% or 7%, and why the 770 is 7% ahead of the 280X (in DX).
At the lower end 260X, 270X the improvements are basically non-existent, which imo is pretty solid evidence that there hasn't been a lot done on GPU optimization.