On Normal mode Jin'rokh the Breaker was killed by 25,429 raid guilds. That's 99.87% of all raid guilds.
On Hard mode Jin'rokh the Breaker was killed by 5,425 raid guilds. That's 21.31% of all raid guilds.
this goes down to Hard Mode Lei Shen who was killed by just 85 raid guilds, that's only 0.33%!
This is clearly overtuned content since even vanilla Naxxramas was cleared by 3% of the raid guilds according to Blizzard iirc.
For comparison hard mode final boss kills:
H: Madness of Deathwing: 14126 (22.05%)
H: Ragnaros: 7945 (11.53%)
H: Nefarian: 3560 (4.71%)
H: Halion (25): 969 (7.99%)
H: The Lich King (10): 5664 (6.71%)
H: The Lich King (25): 1101 (1.85%)
And we could all agree, that the LK 25 man was also overtuned. And even he has 500% more kills than Lei Shen.
I think they need to nerf the heroic and normal content a little bit more or they will lose the remaining raiders in the long term.
The number of raiders has dropped from nearly 90,000 during WotLK to 25,000 now!
PS: source
http://www.wowprogress.com/