pickups have alot to do with sound...i remember one of my humbuckers sunk in and i couldnt make a sustain splashing bleach on black clothes...hahahah...anyway...it also helps to have good distortion......i never considerd string size an issue....i play with 9's...and low action i never noticed a problem.....i can mush the trem bar down to the body and still get a super harm.....just check your pickups and make sure your not hitting the harmonic dirctly on the fret....hit it just a little before the next fret