Read the TV instructions on positioning in relation to heat sources.
As a Heating Engineer, all I would be interested in is the safety of the fire and whether the TV, cables or mountings interrupted the safe use of the fire.
Went to one house and she was going to drill from the back of the TV - directly through the chimney breast into the fire flue - to run the cables so that they were hidden. Not a thought to what effect it could/would have on the fire.
Its not a good idea for a couple of reasons - firstly whatever the make of tv its going to suffer over a period of time from the heat etc and will shorten its lifespan considerably. Secondly make sure you would be happy with looking up at something sat that high up the wall - maybe ok if you are 20 feet away from the screen but any closer and personally speaking I know I'd suffer from an aching neck pretty darn quick....