I don't pretend to say it wouldn't be a significant nerf.
But the feat needs a significant nerf, since it is significantly overpowered in the hands of a player who knows what he or she is doing.
Whether it's too much is a different discussion. It certainly is not a good argument for keeping it as is.
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-Prof/+2Prof changes nothing about the feat at high levels, where is it as its most abusable. At level 20, it even gets more overpowered!
And at the lowest levels, where you have the most difficulties overcoming the -5, the reduction to -2 is a godsend.
Yunru's claim is patently false. There has been several threads that use false or incomplete math, perhaps he's thinking of those. But no, I am sure I have personally shown him exactly why the feat is too good, and exactly how you go about abusing it.
For those new to the concept, in short: build a Dex/Con Battlemaster Fighter Sharpshooter Crossbow Expert using effectively twin 120-ft range shortswords (yes, I mean that literally, and can back it up with excruciating detail if needs be) to deal 1d6+15 damage (not counting magic) against enemies with as high AC as ~18 (which pretty much is every non-epic foe) at mid to high levels, attacking always with advantage (easily attained through party tactics), negating most misses with Precision Attack superiority dice. Add Lucky or Halfling and the window for straight misses shrink to almost only rolling 2's during nova rounds (the probability of rolling a 2 is close to one in four hundred, by the way).
This build utterly outclasses any martial build that doesn't use -5/+10 feats, since it outputs massively more offensive, without sacrificing significant defense or other utility, since it's still a sturdy fighter build.
Meaning it ruins the fun in variety, since you no longer can choose "regular" builds with a clear conscience - there simply is nothing that can excuse abstaining from +40 bonus damage per round. This means many cool weapon stances and combos fall by the wayside.
It also completely destroys the game's ability to function given standard encounters and monsters.
The feat absolutely positively needs to go. That's not an opinion, it's fact. Don't let anyone that haven't stress-tested the game at high levels tell you differently.
Just think of the featless game. It works MUCH better, and with a HUGE variety of builds*.
*) Sure, the EB Sorlock needs to be addressed in either case, but let us consider that as a separate discussion.
So the suggestion to remove this feat does not lead to some kind of fringe or exotic game. Unless you can claim the featless game is even more broken. Which it isn't.
The feat needs to go, at least if you have players who knows their way around a D&D charbuild.
-Prof/+2Prof changes nothing about this.