Original thread from 2011:
I finally got around to trying this, with an Oxford 944-endowed FW800/USB3 OWC Mercury Elite Pro mini enclosure, mated to a Samsung 850 EVO SSD, as a boot drive for my 2010 Core i7 870 27" iMac.
Although my maximum sequential transfer speeds are limited to about 80-95 MB/s, which is actually a bit slower than my internal HDD, the random 4k read/writes went from 0.7-1.5 MB/s to 18-32 MB/s. That represents a 20X increase. That's remarkable, esp. considering that FW should add some latency. Despite any added latency from Firewire, this FW800 SSD still is actually faster than older generation internal native SATA SSDs for random read/writes.
1) Boot time isn't blazing fast. I suspect this is because of the mediocre sequential transfer speed.
2) A couple of times I got an unrecoverable beachball. Applications were still open and moveable, but I couldn't do anything with them. It turns out this was due to the drive being inaccessible, and I had to hard reset. I solved this simply by adding an AC adapter. I guess even the supposed 7 Watts provided by Firewire is not sufficient (unlikely) or else fluctuates to sometimes being insufficient or problematic (probable). I'm glad I made sure to get an enclosure that allows an external power adapter.
3) Even after fixing the unrecoverable beachballs, I noticed a couple of times getting short recoverable beachballs. This was due to Sophos anti-virus being active. Deactivating this program eliminated the short beachballs.
Now, running this iMac, it just feels like modern machine running an SSD. No, it's not quite as fast as the current PCIe-endowed SSD Macs, but it is nearly as fast as the older generation Macs with SATA SSDs. Most importantly, it is fast enough such that I have no desire to mess with my iMac's innards. This performance will be more than sufficient until I upgrade to a Retina iMac in the future. Overall OS X navigation (10.10.5 Yosemite) is great, and wake from sleep is near instantaneous. Applications load quickly, and I don't find myself waiting around listening to the HDD drive churn anymore.