The only way to stop this kind of misunderstanding is to push the math.
If you make 3 attacks per turn (as the level 10 fighter I described does) then you will make approximately 24-36 attacks over 2 combat encounters. (I think we can both agree there).
If I use precision appropriately 5 times during those encounters I can expect it to turn 4 out of the 5 misses I use it on into hits. Of those 4 uses, each adds an average of 18.5 damage. That's 74 damage total over 8-12 rounds of combat. That's around 6-9 damage added per round by precision.
For the sake of simplicity, let's assume that the fighter makes 20 attacks over these encounters (they're a little easier than not) which allows us to assume that all numbers 1 thru 20 will be rolled.
Let's also say the AC is 16, and that the players are told this (not all DMs do, in which case it could take the players a round or two to suss out the exact AC even if they are paying attention to the rolls).
The CE + SS + precision makes 3 attacks with hand crossbows each round (not sure how he's reloading them, but I'll ignore that here). As you stated in your earlier post, he has +4 from proficiency, + 2 from archery, + 5 from Dexterity, and -5 from SS, for a total of +6 (55% chance to hit, before precision is factored in). He's 10th level, which gives him a d10 superiority die, and he will use this on precision if he misses by 5 or less (I know you said 4 or less, but that would leave one superiority die unused which would unfairly skew the math against your position).
He doesn't bother with precision for rolls 20 thru 10, because they hit without it. These attacks deal 1d6+15 damage (2d6+15 for the crit), for 207 total damage [(18.5 x 10) + 22].
On the roll of 9, he uses precision and hits automatically for an additional 18.5 damage.
On the 8, he uses precision but has a 10% chance of missing, resulting in 16.65 dmg.
On the 7, 20% miss chance, resulting in 14.8 dmg.
On the 6, 30% miss chance, resulting in 12.95 dmg.
On the 5, 40% miss chance, resulting in 11.1 dmg.
The 4 thru 1 miss completely.
This results in an average of 14.05 per attack.
Against an AC of 20:
He doesn't bother with precision for rolls 20 thru 14, because they hit without it. These attacks deal 1d6+15 damage (2d6+15 for the crit), for 133 total damage [(18.5 x 6) + 22].
On the roll of 13, he uses precision and hits automatically for an additional 18.5 damage.
On the 12, he uses precision but has a 10% chance of missing, resulting in 16.65 dmg.
On the 11, 20% miss chance, resulting in 14.8 dmg.
On the 10, 30% miss chance, resulting in 12.95 dmg.
On the 9, 40% miss chance, resulting in 11.1 dmg.
The 8 thru 1 miss.
This results in an average of 10.35 per attack.
Now we have a second battle master fighter, same as above except that he doesn't use SS or precision. In his case, because he doesn't take the SS penalty, he has a +11 to hit (80% chance to hit). Instead of precision he uses something like Distracting Strike, Goading Attack, or Trip Attack. Let's say he saves one of these uses for a crit (I think that's fair).
He hits on a 5 thru 20. These attacks deal 1d6 + 5 (8.5 dmg x 15), plus an additional 4d10 (22 dmg), plus the crit which is 2d6 + 2d10 + 5 (23 dmg), for a total of 172.5 dmg.
The 1 thru 4 miss.
This results in an average of 8.625 damage per attack.
Against an AC of 20:
He hits on a 9 thru 20. These attacks deal 1d6 + 5 (8.5 dmg x 11), plus an additional 4d10 (22 dmg), plus the crit which is 2d6 + 2d10 + 5 (23 dmg), for a total of 138.5 dmg.
The 1 thru 8 miss.
This results in an average of 6.925 damage per attack.
So against an AC 16, the CE + SS + Precision fighter deals an average of 5.422 more damage per attack.
Against an AC of 20, he deals an average of 3.425 more damage per attack.
Whether or not that's OP I leave up to the individual.
However, there are a few other factors that we ought to consider.
1) This is a white room scenario. Assuming that the DM doesn't tell the players what a monster's AC is, getting within 4 is much harder (particularly for the first round or two). You may even expend precision on an attack that would otherwise have hit. I would assume that in a real world scenario, barring the DM just telling the players what the monster's AC is out of the gate, the disparity would be somewhat less.
2) This doesn't take into account the bonus group DPS that maneuvers like Trip Attack grant (via advantage). Tripping an enemy so that the rogue can sneak attack and not miss is certainly worth a few extra points of DPR. Since the second battle master didn't take Precision Attack, he has one more option than the other battle master (which adds to his utility in combat).
3) The second battle master didn't need to take SS, since he wasn't using it. Maybe he took Toughness for an extra 20 hp. He might have taken Alert so that he regularly wins initiative and doesn't lose his turn due to surprise. Perhaps he took Healer or Lucky or Ritual Caster. Regardless, I think we can all agree that the feat is worth something, even if we might differ as to precisely how much.
4) This is only an examination of a 20 round combat. However, Frogreaver posited 24 to 36 rounds. 36 is close to 40 so let's assume another 20 attacks without any maneuver dice.
Against AC 16, the first fighter deals 1d6+15 damage (2d6+15 for the crit); 207 total damage [(18.5 x 10) + 22], for an average of 10.35 damage per attack.
Against AC 20, the first fighter deals deal 1d6+15 damage (2d6+15 for the crit); 133 total damage [(18.5 x 6) + 22], for an average of 6.65 damage per attack.
Against AC 16, the second fighter deals 1d6 + 5 (2d6+5 for the crit); 139.5 total damage [(8.5 x 15) + 12], for an average of 6.975 damage per attack.
Against AC 20, the second fighter deals 1d6 + 5 (2d6+5 for the crit); 105.5 total damage [(8.5 x 11) + 12], for an average of 5.275 damage per attack.
For the latter 20 attacks, the first fighter deals an average of 3.375 more per attack against AC 16.
Against AC 20, he deals an average of 1.375 more per attack.
If we average the damage increase over 40 rounds:
The first fighter deals an average of 4.3985 more damage per attack against AC 16.
He deals an average of 2.4 more damage per attack against AC 20.
Now, it can't be denied that GWM and SS should have SOME benefit. They are feats after all. Is the above too much? Do the additional factors listed above sufficiently mitigate it? As I said before, I think that's up to the individual. In some games it may be, in others it might not.