Well, considering the 1911 is a 90+ year old design vs the USP, a considerably more contemporary design (maybe 10 years old?), it's like running a current Cadillac STS vs a vintage Caddy, of say, 1919. Hot rodding older cars is a lot easier (and different) than hot rodding the new, hi-tech vehicles on the road today.
I have a USP45C, and I bought it for the same reason I bought a BMW - It's ready to go right out of the box. As with the Bimmer, you can upgrade the springs for heavier loads or specific duty in competitive events, add lights(/sights) for night work, add other 'personal' touches, but basically, you're paying a premium for something that offers comtemporary/high technology and is pretty well tuned and sorted out to begin.
Sorry for the goofy car analogies, but clearly, if you like to tinker and tune, the older equipment offers more opportunity for 'improvement' and personalization, but that's the state of the art, then vs now, not to mention design tempered by product liability considerations and 'regulations' that didn't even exist in 1911.
M2