Shadow rear derailleur is simply the shape of the derailleur cage, it doesn't change the cable pull. I use 9sp (mtb) shadow rear derailleurs with road shifters all of the time.

Perhaps you're thinking of the clutch mechanism that Shimano and others added to the 10sp mtb rear derailleurs? The clutch helps to reduce / eliminate chain slap against the chainstay when going over rough terrain. As mentioned above, Shimano 10-speed road and mtb rear derailleurs are no longer compatible, so your idea of mounting XT shifters on a drop bar is an interesting option for that. Though I personally prefer using the Microshift BS-M10 bar end shifters for that application.

Mike