Wow. I'm impressed. You should be getting pretty good with that thing. (I have one, and I agree with those who say, "you can be fast or accurate, but not both." You might prove us wrong!)
1) Have you tried a Hogue Handall? The finger grooves may let you shift your hand down slightly, which might be enough to reduce the battering. Its cheap, and if it doesn't help, you're not out much.
2) You might try one of the 15 round mags, with the Kel-Tec addon plastic spacer. This gives you more grip to hold onto and that, too, could subtly let you reposition your hand without damaging your ability to hit what you're shooting at. (The longer magazine with insert make the grip, in effect, into a longer grip. It great improves my accuracy when firing fast, but since I don't experience the problem you experience, I'm not sure it will help with the battering.)
http://www.kel-tec.com/p11-accessories-2.html
CDNN had the S&W mags with spacer already installed recently at a very reasonable price. I think this combo makes the gun a lot more shootable.
(And if you haven't done so before now, you ought to check out
WWW.KTOG.ORG.)
[This message has been edited by Walt Sherrill (edited October 06, 2000).]