For levers run teh new ASV super shortys if you aren't going to run handguards. They are awesome and you can get them SMALLLLL
-2 should be fine to ride with but watch that the sprocket doesn't get so small the chain rubs teh slider constantly. Better off going like -1 front +2-3 rear and keep stock chain length
I personally run renthal fatbars and LOVE them . They are the 1 1/8" bars and work great for stunting. I have laid mine down HARD learning circles and haven't twisted them yet. The other ones to check out are the protaper contours. For a bend get like a CR Hi, or KTM bend. They are pretty flat and stay out of the way when doing like seat stander to special K or high chair.
Call Fullthrottle Inc. They have a specific handbrake kit for the 400sm. I have a handbrake on my YZF426 and I love the thing. Mine, however is all custom stuff adapted to fit. The other thing to check is the new CABLE OPERATED sm setup from full throttle. If I wreck my front master thats what I am converting to.
For tires rock a Pilot Power front (only tire that is worth a damn for repeated street stoppies) and a Pirelli Diablo Strada rear. It is a SUPER long life tire that grips great. The added life isn't with a harder compound but because the tire is THICK. I run that setup on my stuntbikes and it's perfect. As a side note to how good the pilot power is on the front. I talked to Kane at stuntfest this past summer and he STILL runs a pilot power front even though he is sponsored by maxxis. He said that until they make a front he can trust he will always run a PP