GDDR5x is used on literally one card... yet has become the memory to use, even though HBM2 is on two cards(two supposedly available.... but no one has actually seen a working GP100).
This is a fact, if 1080 used HBM to achieve it's bandwidth, it would be in the region of 150W instead of 180W and it would be dramatically harder for AMD to beat it's performance/w with a gddr5 RX480.
If Nvidia wanted to give 1080 512GB/s of bandwidth, what with it losing ground to Fury X and 980ti at 4k compared to 1080/1440p, it would have to increase power by likely another 20W or so to achieve it, HBM wouldn take an extra 5W to do that.
Using an older architecture, the reason the Nano could beat Nvidia on performance/w against a newer architecture, was purely by using HBM.
Whatever the power limit on GP102 is, Nvidia will run into the situation where the memory will use a lot more power so the gpu core can use a lot less power within a reasonable single card wattage. Again HBM allowed a Nano to match a newer architecture in performance/w with an older one. Now think Vega, newer than Pascal realistically with HBM vs Pascal with gddr5x which uses significantly more power at any given level of bandwidth. AMD is going to have a HUGE advantage.
We also both have some supposedly leaked specs, it's funny because when you were using GP102/GP104/anything Nvidia leaked specs it's fine, but leaked Vega specs is a no because you say so.
From GP106 > GP104 die size, from most generations gaps in die sizes, from RX480 die size it's incredibly reasonable to presume small Vega will be a bigger die than GP104, and based off supposed architectural efficiency of Polaris without HBM, it's extremely naive to believe a larger Vega core with HBM won't beat a 1080.
Leaks put it as a circa 4000 shader part, it will probably be 350-400mm^2 both because of that shader count, the need to have a decent gap between it and RX480 die size/shader count, the need to have enough shaders to utilise HBM2 bandwidth and because leaks put it at 4000 shaders. All of that is errm... an idea of what Vega's specs are, and all of those ideas would have it comfortably ahead of the 1080 and what's more is if it uses HBM2, it can probably do that in the same power as the 1080, if it uses gddr5(x) it will likely use a bit more power but maybe still have better performance/w.