I know that when kitz compiled the list, the recommendations came directly from forum users (mostly on ThinkBroadband forums) who posted the values that seemed to work best for them.
RWIN should be an exact multiple of MSS, and I think the TCP/IP stack in the operating system adjusts the RWIN to be a multiple no matter what you put in. It certainly doesn't make any sense not to set the RWIN to an exact multiple of MSS, as that would decrease efficiency of transmission, almost certainly negating the effects of any tweaking.
You're right about Window Scaling - above 65535, Window scaling has to be enabled for it to work.
I'm not sure about Time Stamping, and there doesn't seem to be a definitive answer. It might be worth having a read of the
DrTCP Tweaking FAQ page which briefly explains what each of the settings do.
Also, when tweaking and testing, try to do it at a non-busy time (early hours of the morning is good), so that exchange / ISP contention has less of a chance of affecting your test results.
Have fun!