As of yesterday when I was playing a 1080 Full HD file on a 55inch 4K TV going through a surround system receiver on Kodi 17 RC3. I noticed on the top right corner of the screen a thermometer icon showing up. Checked the temp of the pi 3 and noticed it was above 80 Celsius. Is this a known ‘temp’ problem?
Using this case by the way:
I can say that my Pi3 is about 55 Celsius while playing 1080p on a 1080p TV. 80 Celsius is a bit high, and the Pi will throttle the CPU at 85 Celsius. I did in fact add heatsinks and there was a small change of about 3 to 5 Celsius cooler running.
Are you running Kodi 16 or 17? And I’m not sure if 1080p TV or a 4K tv makes a difference in overal heat.
I can’t confirm if TV resolutions make a difference in heat, I’m running the latest version of Kodi 17 
Oddly enough, my current pi temp is between 50/55 Celsius (and not even playing anything! Just being in the main OSMC menu)
I just checked my Pi3, It has been idling for about half an hour now and it shows 47 Celsius. Between 50/55 Celsius idle is a normal temperature I would say. Keep in mind there is a lot on that little board without any active cooling or heatsinks, add a enclosure for the Pi and heat builds up.