I'm running the 1080ti's and they all ROI'd in what I think was a reasonable time.
Not going to buy many more before Volta comes out. Maybe you'd be best doing a split, spending half of the investment now on Vega/1080ti and learning. Then depending on which flavor you prefer, go with Volta or more AMD cards.
That's an interesting idea. We do need to how volta is going to perform with mining. But It's not like I'm going to set it up all tomorrow. It's a very large operation so I think the best thing is to start small and see how it goes. Plus I need cheap power supply and cooling mechanism.