+1 on the Rick Ritter suggestion. I have bought 2 exhaust kits from him for out 250 trailblazers; he will set you straight on that! Google "ritter cycle". He is in OH or IL i believe.
He will probably tell you to put aftermarket piston in the 400; they are known for breaking piston skirts.
You may also consider converting to premix and using polaris gold oil; we've done that on all 3 of our Tb's and love it.
FYI the 'airbox mod' is simply removing the limiter in the airbox lid snorkel. You can do that before any mods
2012 Sportsman 500, Red.
Highlifter HD Springs all around, Moose pregressors on front (not activated). EPI 'patv1' primary clutch spring, UNI air filter w/ PJ-1 oil. K&N Model KN-303 oil filters. Bikemaster foam grips.