The problem is the Nvidia fans flat out refuse to believe AMD can pull of such a thing, they need some justification to lay against the cost of the TitanX, so they choose to ridicule the very thought that those specs are even remotely anywhere in the same country as the truth.
The fact is only AMD know the power of the card and its potential with HBM, everything else is merely speculation, until we see actual specs of the card and then true benchmarks it's all just theory and conjecture.
I will say once again though I believe HBM to be a game changer, Nvidia are planning to use it, so while I am skeptical I also will not be surprised if the 390x is near the quoted figures in regards to performance, I also expect it to come in around £600 or so, I have no evidence of any of this however I have a hunch AMD have something huge up their sleeves, if not then I'm sure you would have seen specs released by now.
All these "leaks" are fuelling the hype train, something AMD normally don't bother with as lately their cards haven't really been overly special, but like I said a while ago I just think this time they are holding the trump card with smug "you ain't seen nothing yet" grind on their faces
I could also be terribly wrong tho as well lol
The thing is that AMD fans get so butt hurt about what nvidia fans think. Which is what is just so dumb about the whole thing because really who cares? Fan boys suck deal with it. You don't have to justify your purchase to anyone but yourself (and maybe your wife).
I imagine many AMD users don't think AMD can pull it off they all hope that AMD can most nvidia fans secretly want it to do well too because it will make nvidia up their game which we already know they can do. We know they sell gimped cards because AMD let them.