What OS are the machines ?
I woudn't go too far with the TCP Optimizer connection speed slider on a LAN (even gigabit) because the latency on a LAN is very low, unlike a broadband internet connection.
Only the RWIN (TCP Window) value is affected by the connection speed slider. This buffer is a product of latency and bandwidth (bandwidth delay product, or BDP), there is a calculator of the approximate RWIN you need here: http://www.speedguide.net/bdp.php
Keep in mind that Windows 7 will auto-tune the RWIN value. You may have better luck editing your network card preferences and allowing large packets (~9000 bytes) for gigabit.