Anecdotal, but I should mention that my exit node's throughput has increased massively after updating to OpenBSD 5.8. Probably 300+%. Updating to tor 0.2.7.5 seemed to help a little too. My Ethernet driver is em(4). This is the one running on a 100 Mbps link with a budget, low-power Atom board, so it's usually CPU- or cache-bound.