A note to self more than anything.
While I was rebuilding my HP N40L Microserver storage to use new drives and ZFS I experienced very slow (<20MB/s ) First I thought that it was because the source was a degraded RAID5 and performance was lost due to parity. But when the target was a network share instead of the local ZFS pool write speeds went to a more respectable 60-70MB/s.
Since I use more than 4 drives I used the modified BIOS available for this machine that enables lots of extra options, including switching the ODD SATA port to enhanced. I entered the bios and enabled write caching.
Sure enough speeds went to ~100MB/s