The player is choosing to do that though in a way that negatively impacts the game experience for others. That is the underlying problem that I see. I would wonder what's the next mechanic he or she would exploit after I changed this one. The behavior is what I see as problematic.
Taking a feat that grants a damage bonus with a penalty to hit, and then using a spell (or other simple mechanical) device to obviate the penalty, isn't some sort of corner-case abuse of a rules loophole. It looks to me like playing the game as it was designed.
In other words, I don't understand how it is
exploitative for a player to build a PC according to the rules of the game, and to use those mechanical elements that the game provides in order to increase chances to hit.
I don't believe in any expectation that characters are equal in damage output. This doesn't mean they can't contribute meaningfully to combat.
I'm simply pointing out that there is no objective problem with these feats or the like. They're only a problem in your game because of your group's approach.
The game is clearly designed taking damage output seriously as an aspect of mechanical balance. One can see this from the spell rules (eg damage scaling, dice formulae that are somewhat counter-intuitive relative to earlier editions of the game, etc); the way the extra attack class feature is allocated across classes; etc.
(To say that it is
an aspect is not to say that it is the sole aspect.)
Suppose the feats under discussion just gave a flat +10 to damage (with no penalty to hit). Would that be an "objective problem"?
If not, then I think the notion of "objective problem" has become unhelpful. Whatever sort of "subjectivity" you thought the problem of flat +10 to damage feats would involve, it would still be a problem that many players of the game would have to deal with, because of the dramatic effect it would have on the damage output of characters built with or without the feat.
If yes, then the presence of the -5 penalty to hit doesn't necessarily cease to make the problem an objective one. The game provides many ways to reduce or eliminate penalties to hit. Given the number and intricacy of those methods, the game clearly expects players of it to engage with them. (In this respect, 5e continues the "puzzle-solving" approach to game play that is found in 3E, 4e and in the spell-casting elements of earlier editions.)
If the point of the feats is to grant certain characters a modest damage boost, dropping back to a flat +2 to damage would seem an improvement. The +1 STR/DEX option is a variant on this, although it imposes a stricter cap.
If the point of the feats is to grant certain players the thrill of an occasional damage spike, then it turns out that a -5 penalty to hit is not an especially effective technique for rationing that spike, because other aspects of the game which players are invited to explore and make use of make it relatively easy, at upper levels, to break the rationing method. The 1x/turn option is a more effective way of rationing, although might lack the element of thrill. There are other rationing techniques that could be use too (eg 13th Age's natural roll triggers).