Yes, the CS-6800 11-28 and CS-R8000 11-28 are interchangeable.
If you wanted to go to a 30 tooth cassette you can probably use your current derailleur as Shimano is rather conservative on the spread of what their derailleurs can cover... testing is the only way to be 100% sure.
If you would like anything over a 30 tooth cog, on the back, you would need the R8000 11-speed rear derailleur GS.