around here (at sea-level) the Honda dealerships sell new 400EX's with the WB slip-on pipe, and do NOT rejet...
if you ask them, they will say that White Bros. does not recommend it.....

I've never ridden a 400Ex with a WB slip-on, and I wonder if it really makes a difference....

I removed the quiet-core insert and spark arrestor from my stock 400EX exhaust... kept the end-cap, added a K&N air-filter, trimmed both ends of the snorkel, and went up 2 sizes on the factory main jet.... and I CAN tell a difference.... although not much...

but I got the K&N given to me.... so it only cost me $4 for the main jet....

I don't think it's worth the trouble or the money to mess with a 400EX unless you're going to remove the airbox completely and go with a full exhaust system