I should note that, as Steve Bellovin reported, I've gotten roughly
the same performance improvement simply by increasing the socket
buffer size.  Of course, this can be a waste, but waste is always
relative to reward. ;)

As I said before, I haven't tested this systematically, but with
ftp/http transfers, it seems that I don't notice much benefit until I
hit around 1MiB/s.  At ~2MiB/s, downloads from a site nearby seem to
run on average about 25% faster.

I wonder if there have been any reports of this feature being abused
as you describe on FreeBSD hosts for e.g. DoS attacks.

There is also some interesting stuff at
http://www.psc.edu/networking/projects (the corresponding FTP URL has
tarballs if the link to the file you want is broken).  There's lots of
code for NetBSD, albeit from around the 1.6.2 days.

--Blair