Hi Moritz,
im running a few high bandwidth relays, and i'm also interested in tuning the performance - at the moment i have experienced following things regarding AES-NI acceleration:
With an upgrade to ubuntu 13.10 x64 there seem to be no more support for aesni module - so it doesnt seem to be usable any more. With older ubuntu releases it works.
I've recognized that some configurations are running easily with an higher throughput - but could not figure out whats the reason for it.
My VPS'es are almost running in an KVM environment - but the 2 fastest ones running on OpenVZ hypervisor. Processor and Hardware underlying are not identical (corei7 or Xeon E3 in this case) so this doesnt seem to be a reason though for the different speed.