what is limiting the maximum FPS for the 1D X?
is it the bandwith/processing power of the DIGIC5+ CPU´s?
or is it the mechanical shutter construction?
could a dual digic 5+ powered camera deliver 10 FPS for a 21-24MP sensor?
To return to the original question, the 1D X as far as I know is beyond any SLR before in terms of frame rate for a moving mirror at 12 FPS, and also beyond any DSLR's data throughput at 216 MP/s (18MP 12 times each second, it's max in RAW mode).
While both of those are pushing the boundaries, as to which is forming the actual limit, only the Canon engineers will know. However, it's likely the mirror assembly is moving as fast as they could get working reliably at that price point, and the data throughput of the dual digic 5+ at 12 FPS would have been pushed too far with one of the many 20+ MP sensors they have put in every other full frame body released since the 1Ds mk III.
Therefore they're probably both right up at their respective limits, in balance with each other.
To answer your last question, based on the above assumption of an upper limit of 216 MP/s, at 10 FPS a dual digic 5+ camera could work with a sensor of up to 21.6 MP. Any more MP and the frame rate would have to drop further.
And the rumoured 46MP camera with the same processing pipeline would max out at 4.7 FPS - if it's 50+ MP, it'll be 4.3 FPS or less.