Speaking as an Insta player, I also feel like the game has changed alot, just not sure if it's for the better. I'm the same as the OP, I'm better than average in UT2k4 but just can't get the hang of UT3. I've played in clans of different skill levels since UT, and was really looking forward to UT3.
In Insta games, normally you see someone on the other side and he sees you, within seconds he is dead or you are. In UT3 I've found myself running around in little circles, shooting constantly, and he is doing the same, sometimes another blokey joins in and then it's 3 of us dancing around shooting the place up, but still not hitting anything. Admittedly, I can still get some good shots off, but normally end up doing the UT3 dance.
I've also specced some really good players, and again they do the little dance, not as often as me but they still do it. Also I find it hard to tell where the shots are coming from. Again I'm getting better the more I play it, but it just doesn't have the same appeal as all of the other UT games. Out of all of the games, I did prefer UT, then UT2k4, so as I'd heard the movement was supposed to be more like UT than UT2k4. (I could never do the fancy jumps under pressure, so I never bothered with them, which lead to me being called a n00b because of the way I moved all the time, and a cheat when I made some good shots, again because I didn't do the trick jumps, I shouldn't be able to make any good shots). I couldn't wait until UT3 came out, and I had it on pre-order for years (or so it seemed).
My advice is to stick with it and it, and you'll get better. Also it would help to get more people on the servers and keep them there, then we all can improve.