Standard magazine on a 45 Compact is 8 rounds, so no problem there.
Accuracy: I can shoot a group under 1" with G30 at 7 yards with no effort. It took some effort to get a sub 2" group with the USP45c. I'll grant that I am not yet used to the gun and the sights were not regulated for me, but I found it surprisingly hard to shoot accurately compared to a G30, even in single action. Double action on a USP is gritty and hard and long, but I don't think it would get much use, as cocked and locked carry is possible. However, the USP is likely capable of better accuracy than my initial results would suggest. The full-size USP was certainly much more accurate than any of my Glocks.
Pretty much any +P ammo will choke my G30 (3d gen) and G21 (1 gen, rebuilt recently). 185gr will dive in the magazine, 230 would lock the slide on recoil. Any +P was also unpleasant to shoot. USP is much a softer shooter.