Hey dn0
the only thing you can try to "fine tune" is the TcpWindowSize and GlobalMaxTcpWindowsize, make sure both are the same values....
as you can see on picture below, with this RWIN calculator, the settings is based on a ping of 305 ms
and 500 KB/s = 8x500 = 4000 Kbps = 4 Mbps
try changing the ping value in RWIN calculator, it should be between 200 and 310
If your calculated RWIN is below 65536, you need to set Tcp13230pts to 0
you find the calculator here http://www.dslnuts.com/bitsbytes.shtml
good luck
VanBuren