I have an XD in .45GAP. The only thing stopping me from getting rid of it to get an XD in .45ACP is the fact that it shoots so darn good.
A lot of people say the GAP is headed for extinction. They've been saying that for over a year now. While Springfield may have shot the GAP in the foot by creating the XD-45, their new EMP will more than likely restore the GAPs usefulness.
I like the GAP round. I don't see the ACP having anything over the GAP. However, I do see the GAP having at least one thing over the ACP and that is it's thicker case webbing. Once more data is available, I think the GAP could become a reloaders dream. Some people will tell you how hard it is to get it or how much more expensive it is, but chances are, they don't have one. I pay the same amount for ACP and can find it just as readily as ACP.
But god forbid we mess with all-mighty .45ACP, expand our horizons, or move to new technology.