Replying to myself:

> On a sufficiently new kernel, you could put this in /etc/sysctl.conf
>
> net.inet.tcp.sendbuf_auto=3D1
> net.inet.tcp.recvbuf_auto=3D1
>
> to enable TCP window auto-tuning.  This functionality arrived in
> NetBSD-current on August 7 this year, and will not be in 4.0.

Err... August 2, but what's a small week between friends? :-)

- H=E5vard