bigredgrad01

Shimano XT 11-40 cassette works with the stock shadow Ultegra RD8000 GS with just the B-screw adjustment. No need for RoadLink like you would need for 6800. Just needed a new chain, since stock one was trimmed too short for this. Incidentally, the full 116 link chain length was perfect so that RD is not overstretched in big-big and still tensioned in small-small with the 50-34 upfront. Note this means 45T difference between 50F40R and 34F11R, which is above 39T capacity that Shimano specs, but it works fine.
Shifting is fine after adjustment, but of course, it's not going to be as smooth as 11-32 with the bigger gear jumps, especially between the smaller cogs. Also, chain slap was not an issue even on singletrack, I do have the chainstay wrapped with a cut inner tube just in case. The only minor issue is that because the 11-40 cassette is pretty heavy, If you are going fast and spinning in a tall gear (50x11 or 50x13) and stop spinning suddenly, or try to spin back, you feel the inertia of cassette compress the RD spring and then a slight yank on the chain after about a second as it decompresses. But other than that it works as expected. I haven't decided if it bothers me enough to get the Ultegra RX clutch derailleur. I might if I see some killer deal on it, but it's not a huge issue.