Brian all the info on the rifle is in this post, LR Build.
You're right I shouldn't have said 3200 as when I broke that I had pressure signs it was closer to 3150 and 2900 with my loads. Still pretty respectable speeds for the heavies out of a 26" heavy varmint Stevens 200.
The general rule of thumb for an AI cartridge is that a 10% gain in capacity nets a 2-2.5% gain in velocity. With a .243 AI you'll only get around a 10-11% increase in capacity so you're only looking at a 75+ FPS gain at best if the bullet had an original MV of 3000 FPS in a standard .243 Win. However, as mentioned case life is better with the AI because it doesn't stretch as much. I don't fault a Guy for wanting an AI cartridge, but in most cases the velocity gains aren't that great.