[olevelo]

However, 720p isn't supported yet (they said it will come soon), so it will be upscaled either to 1080p or 4k.   

720p works just fine if you whitelist the modes, but like you said wished SD material used 720p rather than 1080p so the TV upscaller can do most of the work. 

Only if you have the Shield set to 12-bit 422 2020.  Otherwise 720p doesn't show up as an option in whitelist (although it does show up as a resolution available in the Shield, under some of the colorspaces).  There's a reason I switched back from 12-bit to 10-bit, but I honestly can't remember why now.  Also, the 720p is limited to 50/59/60 Hz, no 23.98 or 24 Hz.