I haven't tested it with /boot in ZFS, but I have a laptop that has an EXT /boot partition and a LUKS partition with ZFS for everything else (/, /home, etc). It works just fine - and I've never noticed any performance issues with this setup - obviously LUKS has some overhead, but running ZFS on LUKS works as well as EXT4 or BTRFS on LUKS, with all the advantages of ZFS.