I still do not think it's the "phone" that is causing the high heat for some people. I think it's the battery. The battery is a chemical reaction that produces the electricity to run the phone. These devices are built to certain specs that have to be met or they will not function. The software out of the box "should" be identical as it's digital. That leaves the battery which is a combination of various elements and chemicals that interact with one another to produce electricity. If the chemical reaction is off one way or another then you will get excessive heat. If we could just swap the battery I believe this would prove my point. Besides, when we return these things due to overheating what do you think Moto is doing with them to resolve the issue so they can re-purpose them as refurb's? They are replacing the batteries first. If that fixes the issue - done. If not....then maybe they scrap them and right it off of their taxes.
If the battery was defective to cause the heat, it would not maintain full capacitance. For lack of better less technical description, it would be like a short in the cells causing heat to build.
The heat seems mainly due to any app that tries to push 1440p with a GPU that is at it's ceiling pushing math to 1440p. This is why some apps and games do not run hot and others do. Some scale to 720p or 1080p and some do not, since the dev overlooked it or chose not to. 1440p res for a GPU to push on a phone display is overkill X4.
If the battery lasted just a few hours I would lean towards the battery as well, but seems enough anecdotal info now to lean to an over taxed GPU.
The GPU though could be adding to battery heat and that is not good either.
Good examples of apps that stress the MXPE's GPU is VR content (pushes native display res up to 1440p) and the game Guns for Hire. Guns for Hire pushes full native res and stutters badly. It plays smooth as butter on the Note 4 with 805 chipset and the Tab S with an Exynos chipset. Both have GPUs designed to handle 1440p.
Then again, games that scale 1080p are smooth: GTA 3, Vice City, Nova 3, MC3, 4 and 5, Dead Effect 2 are just a few that play smooth. Apps that push 1440p choke the MXPE. Video content though at 1440p does not push the GPU since no real math to calculate. The GPU calculations pushing 1440p is what probably causes the main heat concerns.