How about borrowing something from a totally different genre of gaming, like my wife's Wii Zumba? When she's performing within 90% of max, her character is highlighted in green. When she's between 60% and 90%, shades of yellow, and below 60%, shades of red. There's absolutely no reason a similar, personal system couldn't be implemented to let each individual player know, based on the minimum dps requirements for the fight, how they're individually performing. That provides the feedback you need without the immediate consequence of creating a hostile environment.This is a pretty cool idea. A simple green/yellow/red meter that was boss-dependent and displayed your personal DPS for that boss. If all the DPS stay in yellow or green for the entire fight, the boss will die before the enrage.
Of course, the one thing that might cause issues here is DPS during the execute phase. Most DPS classes see their boss damage jump upwards during the last 20% or so. Accounting for that might be a little tricky. But not insurmountable.
You could make this pretty complicated if you wanted to. If you know the fight, you could draw a damage curve that accounts for phase changes, interruptions, and execute phase. Then just draw the players total damage as time goes on and compare to the curve to determine if the player is ahead of schedule or falling behind.
Still, it's a pretty cool idea, and be enough feedback without needing exact numbers, or revealing performance to everyone. Or maybe the red/yellow/green light should be revealed to everyone. After all, it's not players imposing arbitrary and unrealistic standards on each other. It's the game revealing the basic performance.