I play with a USP.
The problems with reloading CO2 and Paint are usually because the person doing the reloading is trying to hold the marker like they would hold a zeus or a PT. Once you get used to reloading it there isn't really any modification needed.
The USP was the first semi pistol that I really felt comfortable with, so its kind of funny to see Mstrtal fumble with reloading it because he's used to a couple of Zeus.
One thing that I might do this season is to shave down the plastic screw on piece at the end of the barrel so that it will accept an AA intimidator. My only concern is if it will conflict with the CO2 changing.