Well, aside from perhaps upgrading the sights and adding girth/finger grooves to the grip by way of the slip-on you might consider the match trigger installation. It has improved the DA trigger for many though personally I shot my USP40F trigger in even though my normal carry was Condition One. This pistol was/is accurate enough that I replaced my match accurized Colt 45ACP in my SWAT assignment with it.
A threaded barrel is useful for suppressors, flash or sound (provided your state allows it), and an "o" ring barrel may marginally improve accuracy by keeping the barrel locked up. I'm not sure that these are worth the monetary investment.