I used that title line to try and get the attention of ANYONE at Greenbytes.
I have run both Zevo (1.1.1) and the free opensource maczfs (MacZFS 74.2.0). I realize that the maczfs code level is back level a bit but ...
On Mountain Lion the memory used by Zevo is 40% - 50% higher - both in wired and used pools.
Even lower usage is shown by the Linux and FreeBSD zfs implementations(on a per GB of disk basis).
I did some benchmarks on a 60GB USB external drive and in a 8GB mac mini the difference is significant enough to make me decide to stay on maczfs for my Mac needs until someone gets us an answer.
I have also noticed but not measured a significant difference in performance - especially when deleting folders with many sub-folders ... Zevo seems to get all tied up in knots doing this ...