Hi Ex
I'm in the UK too. This device you mention, is my understanding correct that you are using that because it makes it easier to work with the Teensy 3.0 than connecting the micro USB cable? However I don't have to have it? Sounds like a useful purchase, just trying to get my head round it.
That performance sounds fine, I would only be looking at encrypting a small packet of information which it sounds like could be done very quickly. My preference is RSA encryption but due to performance I'm not sure how secure it will be because of the concessions I need to make. So at least this AES should definitely work and maybe I could combine it with my potentially weak RSA. I would probably look to add some kind of initialisation vector to the AES code if there isn't one already.
One of the scenario I'm catering for is that the device itself has been compromised. With AES the private key could be swiped. With a public key it wouldn't matter.
Cheers
Andy