I started to post this in the Glock 43 thread but didn't want to derail it. I've mentioned this in the AR section before but I think it holds true to pistols as well.
In my experience from shooting box-stock to highly modified guns and watching other shooters do the same, I'll offer this....There seems to be something like a bell curve with the level of experience a shooter has and the benefit they'll receive from gun modifications.
At the far left you've got new shooters or the people who might have trouble hitting a bucket at 10 paces. The benefit of gun modifications for those people is minimal. If they're not properly aligning stock sights, they'll probably ignore improved sights. And they'll probably jerk the piss out of an improved trigger the same as a stock trigger.
As a shooter improves the benefit they'll receive from gun modifications will increase. When you know what to look at with your sights and what to do with them, a more refined sight picture can benefit you. When you're able to maintain some control over a stock trigger, a modified trigger will allow you to maintain more control and manipulate the trigger faster without disturbing the sights.
But the better you get, the less benefit you'll receive from gun modifications. Then you'll start down the right side of that bell curve. IMO, most people never reach the point where they'll outshoot a moderately modified gun. Most shooters don't have the time, money, or desire to invest either, to perform at that level. The low hanging fruit is gone, you're back to square one and can't buy the improvements anymore.
At the far right of the graph are the people who are performing at the top. Those guys have developed their personal ability to the point that they can shoot most anything better than most anyone else. They'll still shoot better with their best gun, but they can shoot any gun better than most anybody. I'd guess that there are probably only a few dozen people like that in the country.
