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.We're smitten with the performance of Shimano Di2. Derailleurs using spring and cable tension to move the chain from cog to cog almost seems archaic compared to electronic rear derailleurs, where the highly innovative digital technology of sending electronic signals to a motor to precisely position the chain exactly where it is needed on the cassette. Shifts happen much quicker and with less effort. Shimano even employs a saving mechanism that disengages the rear derailleur so it can handle side impacts without damaging the component. Shimano's excellent E-Tube software allows total customization including shifting speed, the number of cogs shifted with a press and hold of the button, and even what button does what.A note on the cage length: Shimano offers the 8050 derailleur in two cage versions, an SS version that works up to a 30T cog and a medium cage GS version that works up to a 34T cog. The GS version 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.