I upgraded my Shimano 6850 DI2 rear derailleur to the new 8000 series DIs RD-8050 after I found the Ceramic Speed oversize pulley wheel system I bought would not integrate with the 6850. Since I had to purchase a new derailleur I considered Dura Ace as well. Generally Ultegra is every bit as good as Dura Ace at a better savings. The modification of the pulley cage was easy following the instructions from Ceramic Speed. Once installed the new rear derailleur functioned perfectly. Aside from the needed upgrade the 6850 was a fine rear gear changer but I am quite pleased with the new one.
Shimano Ultegra Di2 RD-R8050 Rear Derailleur, name speaks for itself; as This was my first Di2 build, putting the components on the bike is mechanical thinking; pressing the power button and now electrical shifting. I was happy the Shimano instructions allowed me to line-up, adjust the HIGH/LOW Limits, and fine-tune the shifting. The Shimano Di2 components are well engineered and the Di2 system is very reliable.
To Shimano's credit they made this derailleur fully compatible with the previous Ultegra 6800 series shifters so upgrading to the 34-tooth cassette doesn't require a whole new groupset. The shifting is as smooth as I have come to expect from Shimano. With this derailleur and the 34-tooth cassette I don't even think about hills anymore. I can conquer anything above sea level.
I already had the Shimano 6800 series electronic shifting but the biggest cassette you could get with it was 32 teeth. When Shimano introduced the 8000 series with a 34 tooth cassette I jumped on it to help save my knees. To Shimano's credit they made the new 8000 series derailleurs fully compatible with the 6800 series shifters so if you want to upgrade to the bigger cassette you don't need to buy the complete groupet. You only need to replace the rear derailleur, cassette and chain. And the shifting is flawless. Very smooth and fast shifts. You will appreciate the performance.
I've had this for a few weeks and so far it is Flawless. It shifts great and gives you the ability to have an 11-34 cassette which works great in the Denver area where we love to climb mountains.