Doesnt the 400ex have rotary valves? I think those kinda kick ass. I don't know much about them, other that with no rotating mass and springs to compress as in a standard valved engine, that should free up a few horsepower.
A good example of how much a valve train can eat up is the Bugatti Veyron with the W-16 engine.
From what I've read, the total calculated amount of energy in the air/fuel mixture coming in to the engine works out to be 3000 hp. 1000hp is soaked up just turning the valve train, 1000hp goes out as wasted heat, and the remaining 1000hp sees the drive train.