Take a look onto http://www.wowprogress.com/rating.tier12
Take also a look onto http://www.wowprogress.com for nowadays numbers.
If you take a look onto 4 months since release of firelands, it was around 30.000 guilds which killed the first boss.
If you take a look onto ToT now, its 29149 guilds which killed the first boss in ToT.
Firelands got nerfed this point of the timescale, as there was no LFR. ToT will likely not get nerfed for a lot until 5.3 is over.
Infact, we see the same numbers in Cata as like now. If you got other sources, post them.