Not many game-engines support that. Unreal can't do it for example. Your best shot in such a case is to have a low and a highres cubemap (two separate assets) and lerp between or just pick one.
Well I think would make stuff darker, since light is more condensed over a hot spot, theres less brightness around it. Huhhh maybe haha I dunno. (I mean, in reality, looking at the lips example. Obviously in game it doesn't matter, whatever is cheap and fast is good hehe)
I`m pretty sure thats what the exact function of a gloss map *in thoery* should do. The blurred look comes from the surface having a roughness to it. the smoother the surface, the crisper the reflection. This level of surface detail, is usually so small however, that its hard to see in real life, let alone at game res. So…