Daniel,
I thought this too, but the problem is principally the hub drag. To improve rotor efficiency the hub on the X2 has an oversize faired beenie, to minimise the fountain effect (upwash leakage at the hub). This means the hub works best if cruising at a level attitude. Clearly only pusher props will do this.
Apparently Comanche involved a study of ABC vs conventional. Since the ABC concept was without pusher prop it only had a speed advantage of 10kias. The pusher prop X2 also has a faired shaft housing between the hubs, so the whole rotor assy has only a marginal drag increase over a conventional single rotor.
The other benefit of seperate prop is that the prop can be optimised for high speed only. At low speeds the machine can transition like a normal heli, while at high speed MR Nr can be beeped down. The only downsides are cost and weight, although this can be offset by allowing rotor to operate nearer optimum Nr in hover.
Big Sky has gone quiet about X2, so i imagine they have encountered some technical difficulties - that's leading edge engineering folks. Nick sold his soul to the fixed winged one, but will still have much better insight to X2 than the rest of us put together.
Mart