I think you might have mis-understood what I think he might have wanted.
If the client and server support TCP window scaling, there will be a window scaling factor as a TCP option. Re-writing it could effectively shape a client using its own slow start algorithm or one can completely break it. Removing the window scale option on SYN could disable some TCP extensions, optimizing really low bandwidth networks. This is basically a hack to override client side optimization.
You shouldn't increase the window scaling factor on the router unless you know the client has a large maximum receive window... but it would be nice if it could be done anyway
Check this RFC for more on the window scaling factor -
http://tools.ietf.org/html/rfc1323#page-8
A mangle rule which lets you modify any offset of a packet and a condition which works like the wireshark splice option would be really nice for us geeks