Here's the deal. On T14 and T15 we didn't have any percentage nerf, however the content nerfed itself when they unlocked the item upgrades, not to mention the legendary gems/cloak that started coming in about 1-2 months into the tier. Let's face it, item upgrades and legendary gem/608 cloak is the biggest reason why more guilds are clearing ToT on HC than they did the in my opinion kinda easier T14 raids.
However, on 5.4 we will:
a) Get the legendary cloaks on the first week
b) Be allowed to upgrade our gear from week one, which, if you stacked 3k valor is enough to fully upgrade 6 new items on week one
So I see two possibilities here:
a) The content will start nerfing itself way faster than ToT did since we'll have access to the new legendary/item upgrades straight away, making it too easy
b) They are balancing the heroic bosses based on those item upgrades and legendaries, so they'll still be really challenging, and then on the long run the guilds that only recently cleared ToT on heroic will have a much, much harder time clearing the whole instance.
Point I'm trying to make. If they don't decide to take the "just put a debuff that nerfs the bosses" approach this tier and just allow the instance to nerf itself like the previous 2 tiers, we could be looking at one that will rapidly become a lot easier than expected for the hardcore guilds. It's also bad for the more casual/slow to progress guilds since they'll run out of content faster.
I'm quite confident that Blizzard has thought through this and has a solution, but I really enjoyed how challenging ToT was at the beginning of its launch and before everyone got the gem and cloaks etc. and I wouldn't really like to come to the next tier only to say "meh this was kinda easy". Not to mention that for those of us who raid with a competitive mindset, having an easier instance is no fun because it becomes more of "whoever raids more kills first" than "whoever is better kills first". Does anyone know if Blizzard has addressed this concern?
Adding a TL/DR in an attempt to actually discuss what I had intended with this thread
If Blizzard have in mind to allow the tier to be self nerfed by item upgrades and legendaries again, then we risk it becoming too accessible on HC too fast, which has a potential to ruin the competitive side of raiding, while also making the content last less. If they are balancing around everyone having +8 ilvl in most of their gear, then they either need to make a percentage nerf, or we could be looking at a tier very few guilds can clear.