A few of my friends shoot the 625, the 929, and GP100 MC.
The 625's have been sent back for warranty work as have the 929's so I've seen issues both ways. The MC guns, I own one, have been 100% from what I seen.
That being said, the MC have no place in USPSA since you would be competing against 6 shot moon clip MAJOR (625) and eight shot moon clipped Minor guns with your speed loading six shot. USPSA has nothing for the minor six shot to be competitive, you give up way to much in USPSA with that type of gun.
IDPA is best for the six shot speed loader as moon clipped guns have to make major PF.
I just picked up a 929 for USPSA and it shoots great, smooth but heavy stock trigger, accurate, easy to shoot. The one thing that bothers me is that the compositor that comes on the gun is not USPSA legal so it has to come off and most say is fouls up the end of the barrel and affects accuracy anyway so it's coming off.
I'd be interested in a 627 if it was moon clipped but since I shot the 929, the reloads are so much faster with moon clips than speed loaders. That alone would pull me to the 929.