Appreciate all the great responses. This is very helpful. The GoatLink 11, per their website, is compatible with "M8000 (XT) and M9000 (XTR) series eleven-speed Shimano rear derailleurs" and "double chainrings are not currently supported." So it looks like it would not work with my current drivetrain (I think). OTOH, it looks like the RoadLink will only allow up to 11-36, not 11-40 per their website. RoadLink ? Lindarets

Also, my current RD has a maximum capacity of 37T, whereas running the compact crank 50-34 and 11-40 cassette requires a RD with 45T capacity. The long cage version of the M8000 XT RD has a capacity of 47T, so coupling the XT long cage RD with and 11-40 cassette and Shiftmate should work, at least on paper.