do you have a cannon exhaust? if you do cannons have been classified as defect because of straight through design. cop said you got no baffle cos they dont know why, just that theyre a defect. equivalent to being a punched out cat or hot dog muff.
you can increase youre rim diameter by more than 2 inches so long as overall wheel + tyre height doesnt exceed 20-25 mm, whatever figure is. but between you and me if you got 18's with 20 profile you pretty much finished.