The more I read this thread, the more I think: what about weapon proc immunities? Sure, DPS is important a lot of the time, but when it comes to Red/Purple boss fights, it's ALL about DPS. Why?
Well the boss is immune to my wounding/weakening/etc weapons. A CR X boss is immune where a CR X trash mob in another area/quest isn't. Why?
Presumably because the boss fight would be over too soon without the immunity. So throw 10k hp at the boss, make them immune to all but holy/bane damage (and a couple of other effects like destruction and curse) and make DPS the way to draw out a fight. That seems to have been the logic so far. But does the immunity have to be a COMPLETE immunity?
Perhaps on a level playing field TWF SHOULD DO LESS DPS than THF in general- but this would be offset by the increased number of weapon effect procs that TWF could throw at the boss. My dual puncturing rapiers should most certainly occasionally drain a constitution point, but us potential boss-slayers aren't immune to this!
I'd suggest something like this to level the playing field a bit:
* letting bosses have restoration potions/spells/abilities with suitable concentration checks but remove the blanket immunity altogether, or
* making the PC roll an extra attack roll against the boss' AC, or if balance requires it, AC + 10, or require that confirmation to be a 20, or give them a resist roll, or whatever, to get the effect through once in a while - and just as with players the boss stats would regen over time during the fight.
With either of the above, boss' max HP could be lowered along with overall DPS totals. The high attack rate toons could live with their lowered attack rates and even damage since they would actually be capable of whittling down boss stats in a way at least similar to trash mobs.
Think of the elation if someone actually managed to vorpal the boss with the odds of confirming such a thing set to many thousands to one against! "Hmmm... do we do the charm/whittle thing with the Hound, or do we charge it and hope someone gets lucky - but probably wipe?" "Uh oh, the trash mobs have trashed the rest of the party. I'm the barb, the hound will spot me at any moment. Switch to vorpal and CHEEAARRRRRRRRGE!" If it works...
Yes it would need to be balanced/tuned, but it immediately removes the importance of pure DPS party members, while not making them useless at the same time. Worth thinking about?