Riff_raff,
The main rotor reduces RPM with speed. I seem to recall that the design is two speed with autorotation for cruise. Not sure what the scheduling is, but the idea is to hand over power for forward thrust to pusher prop with speed. This keeps the fuselage (and hubs) level to minimise drag. The pusher prop also becomes more efficient as speed increases air mass going through it.
The tip noise on X2 should be far less pronounced than other helos. Normally a square tip produces very concentrated tip vortices, but the profile used on X2 will allow a much larger tip vortex with reduced vorticity. If you look at the just flown A400M you can see a similar idea in its turboprops - the other benefit being improved flow at high tip mach numbers. In a helo much of the noise (particularly during manouvres) comes from Blade Vortex Interaction (BVI) so less localised vorticity helps to reduce "blade slap".