I'm guessing that's not very nice. My ZFS pool was created with ashift 9 (this is 2^9=512 bytes), when it should be 12 (2^12=4096). I will have to copy everything off and back on again.
For everyone who wants to check, and because I couldn't find info on it, run:
If you run "camcontrol identify ada0" (or whatever your device is) you can find out before it is too late:
sector size logical 512, physical 512, offset 0
This is from a lucky drive of course :)