Hello Jouk,
the TCP extended window can be negotiated by endpoints to improve performance on a high delay link
http://www.ietf.org/rfc/rfc1323.txt
see also
http://www.psc.edu/networking/projects/tcptune/
this second link provides procedures on how to enable TCP extended window on several operating systems
the constraint is that both sides have to negotiate this capability at TCP session setup.
Hope to help
Giuseppe