This is going to be more of a test and tune thing for you now. Snaller rear tire will make on revolution fasyerthsn ur 20in tire so u are gaining a small ammount of bottom end. It depends on what you want out of ur setup. I have a 2012 yfz450r as well, tons of mods and shes makin 52hp at the wheels. Way betyer than stock, I also dropped a tire size and to help wil spinning so much out of the gate I slapped on a +1 front sprocket