Android tv boxes have a very powerful graphics. Is it possible to use it for GPU computing?

As far as I know, no project has a GPU application for Android yet.

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I understand, typically smartphones or tablets does not have a great GPU and it is not designed to run 100% time. Android tv boxes are different beasts. There is 4,5 or even 8 cores GPU and these plays 4k videos easily. Yes there is another catch, too many brands like amlogic, mediatek and rockchip.

Even only using the CPU it is more efficient than computers. Example 1 unit takes 4.4W and generate about 1030MIPS. Not the great number for single unit but these is possible to use hundreds and hundreds together. At the moment I using 32bit amlogic S802 but I planning to upgrade to 64bit amlogic S905. It gives another about 45% savings MIPS/W.

The efficiency is known. The problem is typically up front cost and long term durability. I have had ~50 Android devices used for DC'ing over the years and they are certainly not all created equal as you state. However, it would take several years for the "efficiency" to justify the up front expense. Odds are more likely that the device will fail before you see the return on investment. I certainly support the idea of using it if you already have the device, but I certainly wouldn't suggest anyone to rush out and buy them for DC use. The problem you will have with these little devices will be cooling for long term use as most will want to run them 24/7. It would be better to wait for server grade chips to become more abundant before projects put a lot of development effort into the apps.

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Cooling is simple, remove the plastic enclosure and install 14mm x 14mm heatsink. I used nylon spacers, what allows to build about 30-40 units tower. Yes it is still on progress...
I planning to build 4 that type towers and add couple slow speed fans between these towers. Without fan the temp is around 50 degrees. That amount units, any modification can have a big impact for entire project. Example wifi is possible to schedule once every 24 or even more hours. It does not need it for full time. Turn off the blue led can save about 10mA per unit.
And the best thing is that, these are possible to resell as used tv boxes, only keep the plastic enclosures. It means you get almost the same money back as you invested to the system. Just keep the eye on failure rate if that goes too high then it is time to sell the remaining units and start another one.