I bought my USP .45 SS last year. I actually put the match trigger in I sorta made an impulsive buy, it was my first gun. I was to do it all over again, I would of bought the tactical.
But, I wanted to carry this weapon so I didn't buy the tactical, do to large bulky sights, and longer barrel, plus I loved the stainless steel slide look of the USP.
As far as the O-ringed barrel is considered, to answer this question, it would be easier to talk about what the O-rind is actually doing.
For those that don't know, here ya go.
The barrel of USP isn't fixed to frame, so it moves up and down when recoil of action is happening. There's a possibility it won't be in the EXACT same position when it comes back into battery. How exact is the question, and according to HK, this O-ring assures its in same spot everytime.
Its my opinion, that you wouldn't see a noticeable until you were at longer ranges, say maybe at 25 meters.
I don't shoot handguns at that distances, so I'm cool. I love the match trigger though. I can't wait to get it to the range.