Consider the following:
1. Read the manual of your TV, there should be a storage temperature range in the spec. Common sense is around 100–120F maximum
2. Have the fireplace burning for an extended period of time, measure the temperature at a few various points where your TV would be. For example, the outer edge center point and 2 edges of the mantel. Tape a thermometer on the wall also, to read the temperature closest to the wall, and try to measure a few spots betweeen the lowest and the highest point.
3. Then you have the idea of the ambient temperatures your TV will be at while the fireplace is being used. If they are close to or above the maximum storage temperature (say 100 or whatever your manual said), you should not keep the TV above the mantel when the fireplace is being used.
4. Bear in mind, the bulit-in cooling mechanism of your TV will not operate efficiently at high ambient temperatures, so if you light the fireplace and watch TV at the same time, you want to be extra careful. If I were you I won’t do both or at least not for long, unless your ambient temperature readings at step 3 are fairly low, say, in the 70s.
5. If you mantel is deep enough, you can just use the table top stand, without having to bolt into the wall.
6. Although, hanging the TV on the wall is actually a better looking option. Concealing the cables is a personal must for me too. But the whole job is such a pain in the neck, and there is more heat concern.