There are a few other drivers that show up in an apropos search as well, not sure if they're well supported yet.. or if OpenBSD's network stack is well optimized for 10G speeds.
che(4) - Chelsio Communications 10Gb Ethernet device
myx(4) - Myricom Myri-10G PCI Express 10Gb Ethernet device
You should see if you can replicate your setup and artificially generate the kind of traffic you'll be seeing, see how it fares.. and then gauge if a dedicated intelligent switch/bridge might perform better.