I was discussing this with my son some months ago, but not implicated (he's now building race quads). The assumption was that since the frame would be receiving movement information in the X plane, then the craft would not need to tilt.
This ought to improve flight times since one would assume that vectoring to maintain Z and achieve X must be less efficient than providing a separate X drive, while Z is achieved with a more efficient 90 degree (vertical) vector.
The issue, without changing the firmware, is how to single-out the 'X' part of the signal (assuming that X is forward). A Quad or Hex could of course take advantage of the additional outputs (8) of a normal flight controller if a firmware mod was made. (Would these be penta & hepta drones?)
I would be very interested to hear your results and methods.