Should be fine. You could test out the placement using a smaller screen on the VESA mount. The approx picture centre should be about the same.

Mounting a set over the fireplace without a mantle to buffer the heat may be a concern.

17 feet are quit enough. but try to uplift your seating so that it would be easy for you to enjoy it for hours...

buy some small stick-on temperature gauges at the hardware store. Buy at least three to make sure you won't be using a bad reading. (These are not highly calibrated professional thermometers.)

Stick them on the fireplace wall near the area where tv will be mounted. Start a good healthy fire (best in winter :) and let it heat for at least a couple hours. Check gauages. if they are less than 100, then you are good to go and don't need to worry about heat.

Though from what I understand, several vendors will void the warranty if you have your TV mounted over the fireplace.

Also, fyi - most construction code for fireplaces require that no combustible material be used closer to 12 inches from the opening of the firepace.

Don't assume your fireplace is like the one in your neighbors house or your parents house. They can vary quite a bit from each other dependent on construction materials and installation techniques.

Here is a link that might be useful: more expensive on Amazon but these are good

I have gas logs in my fireplace and a 50" plasma TV above. I have never had an issue with heat on the TV and it's been place for 5 years. I will admit that I do not run the logs for extended periods of time because the room heats up so quickly. I also have a mantle between the TV and the fireplace.

As for viewing height, my TV is about 65" above the floor and the seating is about 14' away. It gives a nice angle for viewing while sitting or laying on the couch.

Brushworks is absolutely right. Every gas appliance I have had left sticky goo in the nearby areas, especially above it. The gas furnace is in the basement, no problem there.