You need to preallocate space on LVM2 for storing changes and if it fills, bad things happen. You have write amplification of 4MB per write by default on LVM2, while ZFS just writes what is needed, since LVM2 isn't aware of the filesystem structures. All of the advantages WRT cache are gone if you use LVM2 too. Correct me if I am wrong.

That said, if you really want to use block devices, you could use zvols to get something similar to LVM2 out of ZFS, but it is not as good as using snapshots on ZFS' filesystems. The write amplification would be lower by default (8KB versus 4MB). The page cache would still duplicate data, but the buffer cache duplication should be bypassed if I recall correctly.