Action work also makes a tremendous difference in these guns. It's legal in USPSA, but I dont' know about IDPA. Either way, I don't think any of the allowed changes to Production guns in USPSA puts anyone in danger of getting into an arms race. There really is a limit to how much money you can pour into a Production gun. After a few hundred bucks there's not much more you're going to get out of the gun. It really comes down to the shooter at that point.
The IDPA Stock Service Pistol equipment rules are 95% the same as USPSA production. The only differences I can think of is that in IDPA you can't use heavier aftermarket guide rods or stipple grips. In both games, anything beyond good sights, a decent trigger, and grip enhancement(like skateboard tape) are window dressing.