This is the old argument that Kawasaki & DCC don't know how to build a clutch for a 46 hp motor. The 400 does not use the same springs as the 300, the part numbers are different.I read somewhere that the reason the 400’s clutch slips at high rpm is because Kawasaki used the same springs as the 300 and they don’t clamp hard enough to handle the increase in power.
The ramp function on the 'assist' portion of the clutch provides the majority of the clamping force. The clutch lever has great mechanical advantage to make the clutch slip, that is it's job. With such light lever pull even small maladjustment, a little cable drag or a hand with just a little pressure on the lever will cause slippage, and failure.
Kawasaki Racing at an event out west said the problem is mainly in the US and is believed to be related to adjustment issues. This clutch may not be up to track duty but is fine for street duty. If it wasn't don't you think Kawasaki would have changed something for their next production year.
(hint: they didn't)