RE: Hitting the improvement wall... what do to?
There are some things you can try, some have been mentioned already. But to be totally honest some people are just better archers. There is a huge mental game to archery and personality types have a lot to do with it as well.
Probably the best thing you could do at this point to get better is look for a coach and get really serious about your shooting. Gizmos and tuning probably won't help you much at this point. Some of the best scores ever shot have been with old technology and poorly tuned equipment. They were just GREAT archers. And if you asked them what makes them better they would mostly have no idea.
The sad truth is archery is a skill based sport. Any one can learn to do it, but some are just going to naturally be better at it.
I like to golf as well, and I suck at it. I'm sure I could be better, but I'm also pretty sure I would never be "good" at it.
My advice is if you are serious get some target archery books, take some lessons and/or get a coach.
Paul