I don't have a link, but something similar was done with custom kernels with Redis compiled to run native above Xen - the performance gain was only ~ 13% - so in this case it didn't worth the trouble.

But if you a large HPC cluster, getting 13% more of each compute node definitely worth the trouble.

EDITED: see link in child's post

Not EC2, but is this what you are thinking about?… http://openfoo.org/blog/redis-native-xen.html

That's interesting, though I think my takeaway was that the "performance tax" of the operating system layer is pretty minor, all things considered.