I'm sorry if I was misunderstood - I never meant to say that a 5.5k GS had 50k HP with ICC buff.
Anyway, besides that... here's a video from retail of Marrowgar 10N:
https://www.youtube.com/watch?v=X76JrFpY9dQ. The tank here is a T9-geared Paladin, judging from the looks of his gear (this was before xmog was a thing, so it can be trusted). Marrowgar hits him for, like, 11-13k per swing, at most. Now, I have yet to take a record of Warmane's Marrowgar 10N, but I remember getting hit for way more than that - not even counting Bone Slice of course.
I guess I'll start collecting logs and pics to see if that's the case, but I would also like others to share their experiences if they have any evidence, one way or the other.