A better test is to use something like iperf to see what the underlying TCP limits are like: in the case of something like SMB or AFP, it can be particularly confusing if you haven't adequately accounted for buffering and async writes – I've seen copies which “completed” considerably before the network traffic stopped.

If it is window scaling, adding the following to /etc/sysctl.conf should make a considerable difference by bumping the limits up to values appropriate for >100Mb connections:

    net.inet.tcp.sendspace=262144
    net.inet.tcp.recvspace=262144