Well, this thread has knocked my thinking into shape. Read "Cyclic & Collective" by Shawn Coyle, in particular P220 covering tail rotors in advanced helicopter aerodynamics. Good read.
An important tail rotor function is to provide yaw control. If it is above or below the cg then you get a yaw-roll coupling. The vertical stabiliser can be positioned to perfectly counteract main rotor torque, but tail rotor is by necessity more of a compromise. I've stuck in a line in my earlier post.
Choppie, fly with string centred and see what ball says about trim.
Maxtork, interesting i wondered what was happening on this. 177kias is a good start, i imagine power limits level flight to 165kias. The only information i can find on X2 development is this frustratingly slow update web vid:
http://uk.youtube.com/watch?v=E-rDl8lbVcc