There would be a major speed bump. Not as fast as having the drive on an internal SATA interface but noticably fast all the same.
I ran a four and a half year old iMac 24 inch on a Crucial M4 SSD via a Firewire 800 enclosure for about 5 months to get a major boost over the internal SATA HD.
You'll notice an immediate improvement to random read/write speeds, day-to-day things like app launch and system boot times. Sustained rates will only be as fast as the interface you pick and may not be any better than your internal drive. However since most OS and app disk activity is random this isn't too important.
With Thunderbolt available on your system a future SATA 6Gbps enclosure to Thunderbolt interface will remove the sustained transfer rate limitation.
Avoid USB 2 as you will never reach the theoretical 60 MB/s due to USB protocol overheads. I have only ever seen a maximum of 35-40 MB/s over USB 2 hence my choice of a Firewire 800 enclosure.
In the end I went with the most expensive but also the fastest option: #1 which was the Mercury Accelsior PCIe based SSD hardware RAID array (RAID 0 by default which I will keep) in a Mercury Helios Thunderbolt PCIe chassis. The main reason was because it was the only one that mentioned it was bootable (and made a big deal out of that fact).
I paid $799 for the 240 GB model (it's 128 GB X 2 minus the unusable amount...a nice touch for the seller not to count that). It's got a lot of technology built into the card including processors and cache etc. to monitor and control the hardware RAID array including extra error checking and load leveling/balancing. Also it's theoretically upgradable (each drive is called a "blade" and while they don't yet sell independent blades they may in the future). Also the PCIe chassis is an extra bit of the investment that could live beyond the SSD, just like the whole arrangement could live beyond the use of my iMac itself.
This is a great review on it where they say it's "really fast. Ridiculously fast."
A guy on youtube has a (somewhat boring) video of his Mac Pro taking 3 minutes to boot and launch paint shop pro with his hard drive and then after setting up one of these it cut down to 15 seconds! Crazy balls out fast.
This is the sellers sites for;
Of note is that if they didn't offer the bundle discount and if it were not so much more expensive I might have opted for this much more attractive chassis.
Or for even more and larger chassis with multiple slots and supporting larger sized cards.
These chassis got me thinking about the possibilities with thunderbolt. Now any iMac or MacBook can be as capable and expandable as a Mac Pro...there are even developments with video cards in one of these things! I do wonder if it would be possible to take a MacBook Air and hook up a thunderbolt PCIe chassis with a high end graphics card with the display on the internal monitor?
If I was one of these manufacturers, or even Apple, I would consider making a full on chassis with PCIe slots and storage areas, etc. I really wanted a Mac Pro but they were so old...now I can get some of that goodness piecemeal, though for a price. :-)
I get the device about a week from now and will post back my impressions if anyone is interested.
Best Answer
FireWire 800 is the fastest external connection you have, and roughly matches the 6 Gbps speed of the SATA III drive interface, (though 'real-world' performance may be reduced, and a direct internal connection may be slightly faster).
FW 400 is, as the name suggests, half the speed.
USB 2.0 is considerably slower.
Using anything other than FW800 is going to limit the benefit of having the SSD.
I'm surprised that any manufacturer is producing a drive with USB 3.1 and FW 400, rather than 800, TBH.