I think that among the stock tanks, the 400F's tank is the best looking.
I also LOVE the 400F exhaust and think it's dead sexy. It's pretty much the reason I chose the 400F over a 550. It looks amazing and is more efficient than the 350's 4-into-4s, being (reportedly) responsible for 2-3 of the 400's 5hp over the 350.
Having said that:
The only downside to the 400F's exhaust it the fact that it's not the most efficient design they could have used. Ideally, the header pipes should be equal-length when they get to the collector, and the factory pipes are unequal length (due to design needs and styling). Also the collector box itself is sadly compromised for ground clearance. The angle (shape) of the collector is rather sharp as it angles to the offset silencer port and the exhaust gases from pipes one and two have to be forced sideways into the silencer, which at high RPM, would hinder flow.
The Yoshima exhaust, as ugly as it is, is arguably the best 400F exhaust, but for my build, I'm going to stick to the stock exhaust header with a different silencer.