ZFS write speed performance tuning (NFS)

[Gil Vidals]

I'm building my first ZFS NAS and I'm struggling with write speeds over NFS.
I want to know if it is possible to somehow set NFS to async AND keep
slogging active with my SSD write cache?

Here is a brief run down of my write speeds on a 1 gigabit LAN and FreeBSD
8.1 using ZFS:

WRITE SPEEDS of 1 GB file:
  55 MB/sec: scp copy test
  28 MB/sec: NFS with async (zil_disable="1")
  12 MB/sec: NFS with sync (default settings)

3 x 1TB 7200 RPM SATA HDs
CPU Intel i7
Controller: LSI 1068e
Controller: ICH10R (for read and write SSDs)

This NAS will eventually go into my VMware environment and unfortunately ESX
uses o_sync in their NFS client. AFAIK, there is no way to change the VMware
NFS client, so I thought of turning off async on the server side and yet
somehow still use the ZIL SSD Cache for fast writes?

Looking forward to learning more about ZFS.

--Gil Vidals