Originally Posted by
pesty
From everything I've ready, the 8000 series Di2 is fully compatible with 6800. As others have pointed out, the biggest difference is going to be the rear mech. I was able to put together a 6780 upgrade kit (I already had brakes/crank/etc)with the medium cage RD for about $700 last year ordering from the UK. I swapped the cage with a short cage from a mechanical 6800 and have been extremely happy with it.
Honestly, if you have the money and like the newest/latest/greatest, go with the R8000, but if you can throw together a 6870 and save a lot of money knowing you can add R8000 as wanted/needed, personally I'd go that route.
The only problem with inter-generational frankenbike Di2...Shimano has in the past killed the compatibility via eTube firmware. Not a problem now, but might be in a few years.