I also desperately need a split keyboard. But these are dealbreakers:
* Requires me to learn a different finger pattern than the Apple keyboard (even ones with the Fn where "the control should be" are a problem for my muscle memory). I use the MBP too.
* Requires a lot of vertical movement, i.e. big deeply depressed keys
* Tall keys that require my wrists to hold my fingers in the air, or for my wrists to lay on supports which put all the weight on my wrists instead of my arms.
I have a Kenesis Freestyle for Mac, and I don't use it for these reasons, even though it relaxes my wrists rotation around the vertical axis.