Shimano Ultegra RD-R8050 Di2 Rear Derailleur, GS 11-Speed
Shimano's refinements to the latest Ultegra Di2 group are truly remarkable, and while we always fancy a full Dura-Ace build, we'd be hard pressed to decipher a difference in performance. That makes sense since Ultegra gets Dura Ace trickle down technology, and for rear derailleurs, the RD-R8050 borrows heavily from the firm's top-end Dura-Ace RD-R9150—including the low profile Shadow design. This profile places it closer inbound to the frame and tucks the derailleur under the cassette. This protects the derailleur in the event of a crash and in general from getting banged around, so it remains better aligned making shifts smooth, quick, and accurate. The RD-8050's new geometry allows for shorter throws that result in faster and more precise shifts even over a wider range 11-34 cassette.
The medium cage GS version works with up to a 34T cog. It can be used with Dura Ace Di2 if you want to use a 32T or 34T cassette with a Dura Ace group as that group only offers one short cage rear derailleur option.