If you're running a ZFS filesystem, it's better off if the OS itself can manage the disks. Having any kind of RAID adapter in the way will prevent ZFS from maintaining itself. For one, ZFS will not be able to monitor the SMART status of the drives.
If I were you, I'd pony up for the M1015 and flash it to IT mode. This will give you a very stable controller and will allow ZFS to properly manage itself.
there's no point to using ZFS on top of hardware raid
Don't know about the speed, but here is what I believe running ZFS on RAID would means:
You lose the benefits of atomic writes because now the RAID controller has the last say on when a write happens to the disk. Which means you rely on the RAID controllers NVRAM.
ZFS also may get lied to if the data was written improperly. ZFS would have to take the RAID controllers word for it.
You would also lose repairing files because, from ZFS's point of view, you have a single disk, if the data's integrity is bad, ZFS would have no way to repair it because there is no second copy. (assuming you don't set copies=2)
If the RAID drives fall out of sync the RAID drive may take some time to sync depending on the journaling ability. ZFS will resilver the data it finds bad and at least some OSs may run a resilver periodically to ensure the integrity. Again because the RAID will only display one drive to ZFS, ZFS can't help with the repair/rebuild.
You would be able to expand the RAID (if the RAID has the capability) and maybe rebuild the ZFS data across more drives. (For me not a big plus considering the negatives so far.)
Of course all the snapshot functionality of ZFS would be unaffected (assuming the data doesn't silently get corrupted).
Hardware RAID would almost negate any advantages that ZFS would have. Personally, I wouldn't recommend using anything under ZFS, I would run ZFS on bare metal.
However, if there is an advantage I hadn't considered, I'm open to hearing it.
Q. If one happens to have some server-grade hardware at ones disposal, is it ever advisable to run ZFS on top of a hardware-based RAID1 or some such?
A. It is strongly preferable to run ZFS straight to disk, and not make use of any form of RAID in between. Whether or not a system that effectively requires you make use of the RAID card precludes the use of ZFS has more to do with the OTHER benefits of ZFS than it does data resiliency.
Typically you should never run ZFS on top of disks configured in a RAID array. Note that ZFS does not have to run in RAID mode. You can just use individual disks. However, virtually 99% of people run ZFS for the RAID portion of it. You could just run your disks in striped mode, but that is a poor use of ZFS. Like other posters have said, ZFS wants to know a lot about the hardware. ZFS should only be connected to a RAID card that can be set to JBOD mode, or preferably connected to an HBA. Jump onto IRC Freenode channel #openindiana ; any of the ZFS experts in the channel will tell you the same thing. Ask your hosting provider to provide JBOD mode if they will not give a HBA.