If we are going to talk about the hybrid gap, we need to define our terms. In my eyes, there are two vastly different worlds in endgame WoW: The PUG world and The Raider World.
The raider world consists of organized guilds that have a team of raiders who plays with each other regularly and bring a balanced composition to every raid. All the raid buffs are accounted for, classes are represented fairly equally and a good balance of melee and ranged is achieved. Raiders are scrutinized to a higher level on how they spec, gem, enchant, gear, glyph, and a million other nits that get picked; this occurs when they apply to a guild and constantly by officers and other raiders.
The PUG world is a raid assembled ad-hoc depending on who is available in trade chat and who’s friend knows a tank or healer. There are competent people here and there are people who are seeing things for the first time. It’s a wiiiiiide rainbow of people from those who can play the game well and those who might as well be wearing their pants on their heads. Generally there is an 80/20 rule where 80% of the people carry the other 20% and generally the 20% soaks up the loot. PUGs are not doing progression content, and people are generally doing PUGs in order to gear up for applying to raiding guilds or to gear up and play alts. Generally when I think of the PUG world I am talking about the Looking For Dungeon system, because those groups are the purest random gathering of players where the biggest crapshoot of players can be found.
The hybrid tax is present in both situations, but where the rubber meets the road is in the raider world. Raiders understand more about their class than the average PUG, and can get the most out of their character in terms of damage throughput. In the raider world pure DPS has the edge and can typically put out 5% or so more damage than a hybrid DPS class. Things that can sometimes fluxuate are encounter mechanics, class nerfs/buffs, latency, and player skill, but in general if all other things are held equal, a pure DPS will come out on top.
However in a raiding environment, this gap becomes very small and almost unmeasurable due to random events that happen in any given raid encounter (movement being the most common); because of these random events the hybrid gap disappears for the raider. This is not a bad thing, because in a raiding environment, every player has a very specific role to fill and a pure DPS is just as valuable as a hybrid DPS. A hybrid cannot fill two roles at the same time, because the average hybrid does not have the gear or comparable skill set as his DPS, not to mention the overlapping of raid buffs that would need to be retooled on a fight by fight basis. Trust me, it’s a nightmare when we have to have a DPS caster heal for Sindragosa and fill in the missing raid buffs left behind.
The hybrid tax in the raider world is not all that interesting because it disappears due to the random nature of encounters. Until we get another Patchwerk or Brutalus type encounter, DPS throughput is secondary to survivability and how the player uses the other abilities they possess (CC, dispells). Where I find the hybrid tax the most interesting is in the PUG world.