Stock intake valves have a tendency to cup real bad. We have seen numerous cases of intake valves dropping into the motor. Stock exhaust valves have a problem of the stem breaking off. Stainless valves are a lot stronger. If any dust passes through the air cleaner, it's very hard on the valves.
Stainless valves can withstand it more.
The YZF400 runs lean and hot because it didn't come from the factory jetted correctly. The WR400 is choked down because of E.P.A. Iaws. If you put the proper jetting in it and adjust the exhaust Cam YZF400 specifications, it will run just like the YZF400. You must adjust the fuel sensoring switch or the bike will feel lean off bottom end. Tighten the screws back up.