Personally I think MS should stick to what they know best and let AVs take care of their side of the deal. avast does exclude some areas but that is I believe hard coded into the program settings.
Well this statement "These files are not at risk of infection" is a big blank cheque.
I have never suffered any of the supposed performance problems even on my older system.
If you scan these files, serious performance problems may occur because of file locking.
I would like to hope that MS would avoid having hundreds of millions of users apply these exclusions by passing this information directly to those AV companies that support the various versions of windows. That is the smart way, but there are times when MS aren't very smart.
You haven't previously had any problems with performance or of instability, and that really is the test. Applying exclusions wouldn't cause any issues at the files MS say might cause performance or file locking issues as those files aren't being scanned.