I don't really, and strongly disagree with the brass knuckles form the APG, because the poor to-hit is what balances them. When they have full BAB and can enhance, then they become like a fighter that is using a weapon that does 2d10 points of base damage...
If you compare to the 3.5 Fighter, yeah.. it's too good to be true.
The thing is, the Pathfinder Fighter has Weapon Training bonuses along with his class-only feats. In a one-to-one comparison build (TWF vs Flurry) he more than makes up for the damage loss, and still has higher attack bonus.
Since we are comparing end caps (2d10 damage), Weapon Mastery makes the Fighter immune to disarm attempts anyways.
On top of that, he can decide to go a different route if he wanted, and has additional unique feats that give extra potential (DR piercing feats, apply two crit feats at once, etc).
There's a good thread on the Paizo boards called the "DPR Olympics" that tries to hash out the different classes and builds potential at combat damage. They did a pretty good job of keeping the comparison at a realistic level (comparing vs monsters and not each other, reasonable levels, etc).
The Fighter is fairly universally considered "the best" now at dealing damage, although that's from a pure powergaming point of view.
Giving the Monk the brass knuckles, while a major fundamental shift in his equipment allocation, is not a game breaking problem when put into practice. Keep in mind that he doesn't even get double digit damage dice (say that 5x fast) until he hits 12th level, and most of his gaming life will be spent playing with 2d6 or lower damage dice.
