yeah, heavy duty tube it's realy good idea . i'm not sold to x bushing, i've read that you have to do some metal shaving. do you think changing sag will do the trick?. i'm 5'9'' so lowering bike 1'' - 1.5" will be perfect.
i did lower a little my 450 exc r by changing stock rear tire for 120/90/19 michelin but it's not the case with 19" berg wheel. actually i measured my exc r 18"wheel with michelin and fx 450 berg stock tire and there is a diference. my ktm wheel is 5/8" lower then berg wheel. i think with the stock ktm tire 140/80/18 they will be the same.