You definitely cant compare usage between a ti and fury. The more Vram available, the more that a game will use even at the same settings, regardless of whether it is HMB or not. If what you said is true, every game where a furyx would use more than 2gb (which is practically every game) a 980 would fall flat on its face. There have even been cases of crazy mods making fury hit 4gb limit while allowing a ti to run as normal. By your logic, it should be the Ti to give out first. HMB helps VRAM usage in many ways but not in every way.
I have no doubt the fury comes with enough VRAM but your explanation of using a literal amount of VRAM relative to a non HMB card cant be right. With that logic, the Fury would never run out of VRAM before a Ti and though we know that a fury running out would only be rare, it is definitely not unheard of.
Also if you were correct, it would be proportional irrespective of resolution, where we actually see the nano/fury usage increase just a little going from 1440p to 4k, where as it seems to have very similar usage to the Ti up until 4k.
I dont think you can compare Ti and Fury RAM numbers and come out with any ratio which applies. I dont see why things like frame buffering and such would feel the benefit of the larger bus.