If you're truly stuck with that location, then I say skip that bar. That is unless you're willing to get one of those drop down TV mounts so you can swing down the TV (and presumably the sound bar with it). That might work.
I absolutely loathe high up TVs over fire places, but if there are literally no alternative places you can put the TV or can't do a pivoting mantel mount, then you might be better off with either discrete speakers if you want to pull of Atmos, or just skip Atmos for this application and mount a more traditional sound bar so it angles down toward your ears.