TCP Window Size

Have you never had problem with TCP window size?
In an nv2 link, setting TCP Window Size to 800kbytes, I get 90Mbps TCP on the link (one TCP session); using standard WinXP 64kbytes TCP window size I only get less than 20Mbps. When I try with one instance of Bandwidth Test i get 20Mbps; opening more than one instance the bandwidth increases.
That’s someway obvious but I wonder if I can set the RouterOS to “offer” more throughput to a single TCP session from a PC with standard 64Kbyte TCP window size.
f.

http://slaptijack.com/system-administration/tcp-performance-tuning/

Thank for your answer.
Do you think that the only valuable solution is to modify TCP parameters on client side?
The network I’m dealing with is multiowned and I think It’s impossible for me to change TCP on all hosts.
I heard about some appliances that act as TCP proxy: they essentially break hostA-to-hostB TCP session into three TCP sessions: hostA-to-applianceA, applianceA-to-applianceB and finally applianceB-to-hostB. Appliances “decouple” TCP session using the best TCP parameters on each segment.
I think they refer to as “WAN accelerator” or seomething like that.
f.