Are you sure you're not getting your MBs and Mbs confused? 140-160 MBs (megaBytes) is probably near maxing out your storage system's bus capacity or drive capacity (are you using u160 scsi?). If it really is 140-160 Mbs (megaBits, remember there are 8 bits per byte) that you're getting, it is fishy. You'll never reach 1000 due to TCP and protocol overhead (which increases with transfer speed), but reaching 80% if your CPU/hardware is up to the task is feasible.
I should have noted in the post that if I use to Linux (on the same machines) and do an FTP transfer from one to the other I can get about ~900Mb/s. That's if I write the output file to /dev/null otherwise the transfer gets bogged down waiting for the disk to write those bytes. I'm more worried about just theoretical performance that I can't even get.