since first tuning of FreeBSD TCP/IP stack somewhere in 4.x, I’m typically fiddling with the /etc/sysctl.conf contents from time to time. you know, just to tune stuff.
in the meantime, a lot of things has changed in FreeBSD TCP/IP stack, including introduction of modularity. however, MacOS X that is based on FreeBSD is much more conservative and some options are not available.
therefore, on my MacOS systems I use following /etc/sysctl.conf:
net.inet.tcp.mssdflt=1460
net.inet.tcp.minmss=536
net.inet.tcp.win_scale_factor=5
net.inet.tcp.randomize_ports=1
net.inet.tcp.blackhole=2
net.inet.tcp.icmp_may_rst=0
net.inet.udp.blackhole=1
net.inet.ip.redirect=0
no magic here, it should help you a bit use full throughput of your internet link. generally speaking, those are very small changes and should be safe, but please always consult this with documentation and adjust to your specific deployment.
and enjoy :)