ZFS 4K drive overhead

[Chris]

Hi all,

I'm planning on making a raidz2 with 6 2 TB drives - all 4K sectors,
all reporting as 512 bytes. I've been reading some disturbing things
about ZFS when used on 4K drives. In this discussion
(http://mail.opensolaris.org/pipermail/zfs-discuss/2011-October/049959.html),
Jim Klimov pointed out that when ZFS is used with ashift=12, the
metadata overhead for a filesystem with a lot of small files can reach
100% (http://mail.opensolaris.org/pipermail/zfs-discuss/2011-October/049960.html)!
That seems pretty bad to me. My questions are:

Does anyone on this list have experience using ZFS on 4K drives with
ashift=12? Is the overhead per file, such that having a relatively
large average filesize, say, 19 MB, would render it insignificant? Or
would the overhead be large regardless?

What is the speed penalty for using ashift=9 on the array? Is the
safety of the data on the array an issue  (due to how ZFS can't write
to a 512 byte sector but it's coded with the assumption that it can
thus making it no longer strictly copy-on-write)? Does anyone have any
experience with ashift=9 arrays on 4K drives?

Thanks in advance.