that's not true though. aside from the fact that there is designated gear for plate healers and tanks which should be used instead of tier-pieces, you can do the whole T14 HC content with badly geared tanks (even ~470 would be enough) and mediocre geared healers (~480+). on the other hand, you're going to have a very hard time if your DDs are geared anything less than 485 at the lowest, for some bosses you need 500+. That's the way it has been for several tiers now, gear in the first few weeks is more efficient on damage dealers. the reasons for that are various. tanks can make do with very little gear, now more than ever, because of the current active mitigation system. with healers, you can usually make the fight a lot easier on them by having capable tanks and more importantly good use of raid cooldowns. but you can't really do anything magical to beat the enrage timer, that's a pure gear check, assuming your dps are playing their classes optimally.
what is and was way more important than tank and healer gear are the classes. having a disc priest constantly smiting and/or resto druids doing considerable dps with heart of the wild made a huge difference in many boss fights. cutting healers or tanks doesn't really work during progress because you just dont have the gear or the encounter would be tuned very badly (or you're just playing the encounter wrong).
regards