OSX Mavericks (soon to be upgraded by me to Sierra). In its Network settings you can set the MTU to whatever value you wish, within a specified range. And so, based on what you've said, I've changed it from 1500 to 1492. For such a minor change, I was not anticipating any great difference in 'zippiness to screen' but presumably if there's a better match, the efficiency of getting packets across the Ethernet connection to/from the router will be slightly improved (probably less buffering required). It seems to be running okay with this change.
It shouldn't really make any noticeable difference for tcp, there won't be any buffering saved.
On uploading save of a one off, couple of ms, when you first send a 1500 packet and the router says "too big".
For downloads the router will mss clamp tcp anyway so with or without changing mtu on PC the incoming would be 1492.
I wouldn't change anything on a modern OS that does window scaling - especially rwin type tweaks that may have helped some ancient windows 15 years ago.
I can't remember what Win XP defaults were. If tcp window scaling was off, then for FTTC speeds finding a way to enable it would gain a lot for single thread.
As has already been said - you can do 1500 on BTW pppoe, kit permitting. It's not a ptm limitation just that using pppoe historically meant using 1492 before jumbo/mini jumbo frames were possible on ethernet.