I have been reading BikeForums for a long time but never had a reason to join, but I was very curious on this question, so after finding this thread, I decided to find out for myself since it looks like no one else has tried yet. I work in a shop that sells Trek and we carry the Emonda ALR 4 (which comes stock with the full Tiagra 4700 grouppo). So I pulled the bike into the stand and installed a 6800 derailleur and it works! Beautifully! Even though I was using the Tiagra shifters, chain, and cassette, the Ultegra derailleur shifted quite a bit cleaner than the stock 4700 derailleur did. For the first time in a long time, as a mechanic, I am super happy about something Shimano changed! I have pictures and video on my phone that I'll upload and post here when I get home from work.