[boblo]

Could you help me out, please

I'm running SPMC 17.6a2. Exactly what settings are needed to avoid picture upscaling by the Shield? For example, do the Shield and SPMC both need to be at 1920x1080 or just the Shield set at 3840x2160 and SMPC at 1920x1080? Are there any further specific SPMC settings needed?

There are numerous posts scattered through various fora and sites with contradictory advices. A good summary would be much appreciated.  

Read THIS. In theory, you apply those settings and you don't need to do anything else. Shield TV since SE 6.1 (if I'm not wrong), sends native output resolution to your TV.

OTOH, Have you tested MX Player or Archos Player?