First of all do not put a TV over a fireplace. If you do, you can not use the fireplace. The next issue is the TV ends up far too high. I know this is commonly done, but it is one of the commonest mistakes, and it is a bad one.
The center speaker is quite important unless you have really good speaker that can throw a stable center image from a wide variety of listening positions. An awful lot of speakers are not good enough to do that, and as soon as you get off center the sound collapses to the nearest speaker. The speakers also need good speech discrimination.
The center speaker carries the dialog and so should have excellent speech discrimination without shout. Unfortunately center speakers in my judgement are frequently very poor, as they present challenging design issues.