The maximum a 6870 Med cage derailleur can (officially) handle is a 32 cog.
http://bike.shimano.com/content/sac-bike/en/home/road/shifting---braking/rear-deraileurs/rd-6870-gs.html
From what i’ve read, the main differences between 8050 and 6870 is that the ‘click’ at the shifters is slightly more pronounced and that the R8000 brakes are slightly better. Shift speed should remain the same. So far, i’ve been happy on my 6870 groupset.