I think mid-fi and hi-fi are really non-relative terms. In my opinion, mid-fi is a gear level, and the next step up is hi-end. Hi-fi is the goal for both gear levels, and that is relative. Mid-fi, you spend more than you need to, but less than you could, to get what you want, because you're not satisfied with mass-market sound. If you're buying gear that you really don't need, but it's different, and you can afford it(unlike 96% or so of the rest of us), when you're searching for that last 3% of perfection regardless of cost, that's hi-end.
In my opinion.