-and just have it shred up to 25 magic resist AND armor instead?
The two big problems Wit's faces are its ass-tastic build and the fact that very few champions can justify buying it since amplifying OnHit magic damage has really narrow appeal. With this change its build would still be awful, but eating armor in addition to MR would make it far more appealing to a much wider range of auto-attackers.
IcyPepper1 votesI worry that would make it synergize with 
bruisers a bit much.
Rebonack1 votesI worry that would make it synergize with
Not really.
Flat resist shred stacks poorly with percentile shred.
Big Lincoln1 voteswot, no it doesn't
% penetration stacks poorly with % shred
flat pen/shred and % shred go hand in hand
Rebonack1 votes[{quoted}](name=Big Lincoln,realm=NA,application-id=3ErqAdtq,discussion-id=JG8qvVj5,comment-id=000000000000,timestamp=2016-05-18T23:41:27.181+0000)
wot, no it doesn't
% penetration stacks poorly with % shred
flat pen/shred and % shred go hand in hand
Depends on the order of application.
If the %Shred is applied first, then the flat shred, everything is fine.
If the flat shred is applied first, then the %shred, then you're devaluing your %shred. That's how all resist pen used to work before Riot inverted the order of operations.
Since both items apply OnHit, the flat shred would be (partially) devaluing the %shred. So again, they wouldn't stack that great. Wit's would still be very much a situational item, but its use cases would be quite a bit wider.