As far as a perfect viewing angle the TV above the fireplace is less than ideal like Jim mentions but does make for a single feature point in a room. I have never had a TV fail on one of my projects but we take a lot of care planning this out. Often we incorporate a heat escape of sorts within the design to allow heat to exit the recessed niche. Often we use safe and sound insulation and cement board to further block the heat.
Fireplaces are hottest about 8-20" above the unit. This can change the colour of stone and wreck havoc on electronics. A mantle acts as a heat deflector sending the heat further away from the wall. No mantle and a surface mount TV is a failure in the making I think.
Your manual for that fireplace unit will give you all the answers your looking for and these will help narrow in your design. I like the look of a TV recessed and framed but this often requires shooting out the back of a gas unit or rolling the vent like I did in the fireplace above. With a 'A' vent like shown I do not believe this is allowed nor would you have room to pull it off. These pipes get very hot so heed this stand off warnings and ensure you use a non combustible backer board like cement board, hardi board or Green EBoard around the combustible areas. If possible recessing the mount and getting one that tilts down will make viewing better.
I have tried stirring people away from this look in years past but have come to realize that this is one on the few "Stands" the man of the house can make. So..... I usually push for the new TV and a bigger one, unless of course his wife is signing my cheques.....
We are looking at a new feature wall reno on Howe Street this summer that will feature a two way framed mirror to hide the fact that a TV sits behind it. This is a great look but the quality of the TV picture goes down. I'll post some info on the mirror and the mount once the designer sends it my way. If I have to make it from scratch you can be sure I'll document it here.