Yeah, what he said. The send and receive buffers should be at least 8192. Actually, this just jogged my memory that I had been meaning to test out larger values, and after a short test, it appears bumping the buffer up to 65536 seems to have bumped up my speed by 20% or so.

Right now, here's the relevant line in my smb.conf:

socket options = TCP_NODELAY, IPTOS_LOWDELAY, SO_KEEPALIVE, SO_SNDBUF=65536, SO_RCVBUF=65536

Those other flags are things I've found along the way that were also said to improve performance.