Early planing could indeed be the thing where the shoe pinches.
Especially when pumping is involved to get planing, you can use a lot of lift excercised by the sail and mastfoot onto the board. This is probably where you DON'T want vertical movements of the rig. Once planing though, I can see and "feel" intuitively a lot of benefits.
It's probably a little bit the same like with a moutain bike. When going uphill, a back wheel suspension often causes the back wheel to slide on slippery surfaces with a loss of performance as a result. While going downhill, the back wheel suspension has only advantages in absorbing the shocks.
If anyone has already used the shox.xtr, please share your experiences.