When you say this:
really bad KE coupling.
I assume that you have your CG set back pretty far and the combination of that along with a higher wing loading(due to a heavy engine along with other things) is causing this:
horrible wing rock in harriers, really bad KE coupling.
What servos are you using? how much do they weigh? My guess is that you could cut some weight off it by changing servos. also, are you using a high capacity battery? If so, you probably could use a standard 4-5 cell(obviously a 4cell weighs less than a 5 cell of the same capacity). Hope this helps.
Woops
