I stuck a SSD in my MacBook and put my MacBook HDD into one of these:
http://www.amazon.co.uk/IB-231StU3-...Y1TU/ref=sr_1_2?ie=UTF8&qid=1353318919&sr=8-2
Great enclosure - and gives me full USB3 speeds every time.
No issues at all.
I tested the HDD speed while it was in my MacBook, and then tested the speed via the external enclosure and got the exact same speed with both. In other words, connect the drive through this USB 3 enclosure and you'll get the same speed as if it was still in the system itself.