I went straight to an area where they were discussing the STALKER upd8 that included the 10.1 patchings (and Dx9 patching apparently), there's a dood posting all his benchies for 4xaa, 0xaa, dx10, dx10.1.
On a 4870 mind, all kinds of resolutions (on his 24" screen so he says) which bored the crap out of me after a while.
His conclusion (which are gr8 for those of us with limited attention) was there was a minimal drop in fps 10.1 compared to 10, as u may expect if the card was being asked to do more work tbh.
Guy didn't exactly take wide averages or anything mind u, but his Stalker experience was it was a waste of time.
I don't know from the small amount of natively Dx9 games that are patched to 10(.1) that are ever faster. Unless ofc u don't have a Dx10(.1) card. The cards have to do more work.
U can't realistically dev a game as Dx10(.1) and produce a Dx9 variant. U can of dev 1 alongside the other, and ofc have a choice of which to install at installation time.