Just a hunch but it's quick to test. Try doing this on the client Mac:

sysctl net.inet.tcp.delayed_ack

...note what it reports. It'll be an integer, probably the value 2 or 3...then set it to zero:

sudo sysctl -w net.inet.tcp.delayed_ack=0

...then on the AFP client, unmount all AFP-mounted volumes and re-mount them so that you're establishing new TCP connections. Then try your file copy test again and see if the speed is significantly improved.

I've run into situations where Mac OS X's TCP Delayed Ack algorithm was less than optimal, so it may be good to do this test to quickly rule that out.