I find that hard to believe, simply because of how hard and tight encounters in TK and SSC were. Particularly pre-nerf.
All of those encounters would be trivialized by multiple +15% hp buffs. Hell, mages or warlocks would be able to facetank the silly-looking stalkerdudes they were kiting on Vashj.
Besides. Even if that was how it was, it does not have to be that way now. In my opinion it shouldn’t. Imagine how quickly we’re going to be blazing through future content if we get the green light for stacking multiple DMT buffs. World first 20 min BWL hype?