Can't shift down from 50T with clutch on, but can shift down with clutch off, why?
The RD-8000 is specified for a maximum tooth size of 46 (1x11-46).  Likely you are just exceeding its design criteria and reaping the rewards of DIY mix and match.  

The clutch does not add tension. It add friction in one direction to resist a forward moving cage, but allows the spring to pull the cage back freely.  As the chain drops off a cog onto a smaller one, it has to briefly lift and this causes the cage to move forward against the clutch. The clutch is designed to slow this action - possibly in you setup just enough to keep the chain on the large cog and prevent it dropping on to the smaller cog. A longer chain will make little difference, as the clutch is friction based, so spring tension changes probably won't change the behavior. 

Chainline is probably all you have left to play with. (You don't mention the bike, presume its 1x now, was it always 1x). Moving the chain ring out a millimeter or two might make a difference - or it might not.