Valve float, it's when the valvev springs are not strong enough to close the valve before the piston comes back again. The valve kindof "floats" there for a fraction of a second, and with high lift cams at high RPM, the piston will go down and back up again so fast it will hit the floating valve. Needless to say, that is bad.

Displacement loss isn't significant enough to impact the power output of the engine, while the increase the high comp pistons give will be.