Your idle screw controls fuel delivery at idle and just a bit above. Messing with that isn't going to do anything for the powerband. Screw it all the way in and then screw it about 1.5 turns out.
As for that $400, I'd check everything over and make sure you have everything in order. If you have a main seal that's bad and you don't fix it, you'll wish you'd saved that $400 instead of spending it on aftermarket stuff. I'd check the main seals, clutch, and piston, do a leakdown test, compression check, and do a thorough inspection before anything else. After that, I'd do a plug chop.
If I had $400...
Oil injection removed (free-ish)
TORS delete (free-ish)
UNI filter w/o airbox lid ($25)
Head re-cut ($60? I read that on here somewhere)
Porting ($175-300)
Don't forget that a head gasket, base gasket, and reed gasket cost money as well.
Ask KOR about porting. Also, if you're running any castor based oil, you'd better run it daily. The castor oil attracts moisture and will cause rust after a day or two. Switch to Yamalube or any other high quality synthetic 2-stroke oil.
