Rob,
I can think of one functional reason if you fit a square bottom firing pin stop like the EGW firing pin stops. If you leave the bottom of the firing pin stop square or with a small radius, the slide has less mechanical advantage to cock the hammer, so in theory this will slightly delay unlocking of the slide. I run almost all my 1911s with square bottom FPSs. I do notice a slight advantage in that the muzzle jump seems slightly less and the gun recoils straight back and up without twisting to the left. Here's a thread on another forum you might want to check out:
http://forum.m1911.org/showthread.php?t=13060
The guys in this thread get kind of hung up on how big a radius to cut on the bottom of the FPS- I've left a few of mine square and unaltered and they work just fine.
I can't think of any functional reason to replace the firing pin.