1) Does the advice that later O/S look after RWIN themselves apply to ALL later O/S ?
With
Windows Vista onwards - yes... although you can still run into the occasional RWIN issue.
I have very recently discovered to my cost that nvStreamsvc which is installed by nvidia has some sort of issues with RWIN and can upset your network. The post explaining it is somewhere on their forums, but iirc it has difficultly in being able to calculate the MSS or windows scaling correctly, and
can slow things down quite considerably.
Provided the modem MTU is set equal or higher than the computer's it is not essential that both are the same. BUT how much difference does it make making them the same ?
Erics already explained most of it. I agree leave it at 1500 on the router, then traffic will go through without any need for further fragmentation that wont have already occured elsewhere. MTU is end to end point and all computers/devices should have a default setting even if it doesnt dynamically adjust anyhow. As long as the router isnt set to less you should be fine.
Now that broadband has been out a lot longer, most operating systems handle MTU & RWIN better. Most of the earlier issues have been ironed out and we dont see as many problems as we did 10 yrs ago.
iirc theres only the xbox where you may have to adjust settings on your router. *
Plus - 10yrs ago tweaking your MTU and ekeing out an extra 50kbps was a fantastic result. In the days of 256kb even an extra 20kbps was thought worth tweaking for. These days you wouldnt really notice any difference in a 20-50 kbps tweak.