OpenBSD 4.9 will have send/receive buffer scaling, this was added shorted after 4.8 was tagged.
http://www.openbsd.org/cgi-bin/cvswe...cket.c#rev1.84
For fast connections it might have some impact, using a snapshot may improve performance.. an alternate solution might be to bump
net.inet.tcp.{recv,send}space from their default values of 16K to something like 65535 bytes or larger, this setting will obviously apply to all applications and arguably waste some memory.. then again, most computers have ample.
If you're using OpenBSD exclusively as a router/packet filter, none of what I said above is relevant to your interests.