Parmandur
Book-Friend, he/him
There's significant, and then there is significant. Does it make a difference, sure. So the DM adjusts accordingly: have the party face foes where the Fighter needs to make his weak saves, throw more enemies in, use spell-casting effectively, all within DM control. 5E is not a fine-tuned M-16 with precision-machined parts which requires constant maintenance: it's a mostly wooden AK-47 which will keep chugging along.Just because you want to act like 25% to group DPR isn't significant and that it doesn't change anything doesn't make it so.
Personally, if I saw a player trying to do this, the character wouldn't make it to level 10 alive without some amazing luck and footwork. If a player will metagame, so can the DM.