I really won't suggest it for the time being as the 21" already proved to be a pain in the A**. Here's my solution which is working great for me, it's cheaper and MUCH safer for now. iMac 27" 2012, 3.4GHz i7 Quad Core Processor, 16GB RAM, 3TB Internal HD. I connected a SanDisk Extreme SSD 240GB to the sata connector of a Seagate GoFlex Desk 3.5" Thunderbolt Adapter (STAE122). I have over 260MB/s writing speeds and over 370MB/s in reading speeds. Fast is an understatement but not too happy as I could achieve much faster speeds. I have a funny feeling the culprit (bottleneck) is the STAE122 as it more likely has a SATA II connector instead of a SATA III. Nevertheless, it's a perfectly fine setup for me and lightning fast.
Usb 3.0 could be an option BUT The new iMacs more likely have a SATA III connector in their motherboard and that's 6 Gbit/s - 600 MB/s; USB 3.0 is 5 Gbps so there is a "theoretical and practical" loss of speed. Ideally connecting a good SSD via Thunderbolt would allow to reach great speeds BUT the only thunderbolt adapter I know of that can allow that would be the Seagate GoFlex Desk 3.5" Thunderbolt Adapter (STAE122); unfortunately it appears that even this GoFlex adapter has a SATA II connector which would take the whole setup a step back in terms of speed.