The other problem with the HPA solution is that there's no equivalent to hdparm in the BSD world. ataidle only handles spindown/AAM commands, we need to be able to issue SECURITY and SET MAX commands. You can do this from gparted-live but it would be easier to be able to do it in-FreeNAS. I do enjoy the necessary option flag to use SET MAX ADDRESS though;
--yes-i-know-what-i-am-doing
Back on topic. I'm not sure if I'm doing it right, but so far the iozone testing hasn't shown any appreciable difference between an undersized partition or a full-sized one during "general use."
I also set it up as an SLOG device against a pool and saw some interesting results after copying a huge volume of data. Excuse the mangled graph, but it wouldn't all sit on one.
After a long, steady run at ~40MB/s it takes a hard nosedive. I'm guessing the sustained writes were too much for it, and the writes managed to outpace TRIM. I cut it off after observing that for a bit.