You're right, the most direct comparison is against the 264 Win. Mag.
Craig Boddington once wrote in Safari Rifles about the 6.5x68mm and said,
...the velocities are wonderful-about the same as or better than the .264 Winchester Magnum without the magazine-capacity-devouring belt.
If that's the case (at or better than the .264 Win. Mag), how might it compare to the .270 Win at long range?
Can it use a shorter barrel like the .270 Win eg 22-23" or as you and I suspect, must it have at least 24+" to produce significantly higher velocities to gain an edge over the 270?