Parallels guest OS performance on external volumes

I have an 11" MacBook Air (2011 MacBookAir4,1) with Thunderbolt. I've run Parallels guest OSs from a USB external before, and the lackluster throughput and latency (overall poor I/O) really kills performance. I was wondering if anyone has experience running a guest OS on a Thunderbolt external. Theoretically, Thunderbolt should match the I/O performance of an internal drive, but I've learned the very expensive lesson that theory and reality sometimes do not match. With only 128 GB of space on the SSD, I'd really love to be able to run my Parallels installs on external volumes. Right now, I'm copy/moving them on and off of my primary volume as needed, which creates a significant incentive to *not* use many of my Parallels guests.

Can anyone provide feedback on their experience with Parallels guests running on Thunderbolt drives?