Use at your own peril, it does make a difference.
1. run Regedit
2. Go to \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Paramaters
3. Edit > New > DWORD Value
4. Name it - TcpWindowSize
5. Base > Decimal
6. In 'Value Data:' 65535
7. Click OK
This will force the window size to 64K - the largest possible window size. This seems to give the best performance for TCP/IP data over EVDO.