There is actually some research out there on the use of digital displays in GA/light aircraft, though I'm not sure how much is published as of yet. jellycopter touches on what is turning out to be a big issue - when using the analog systems, pilots were responsible for developing their own mental model of the flight environment (i.e. waypoints, terrrain, etc.) but the new systems appear to allow pilots to offload the maintenance of these models to an external actor (the digital nav system), thereby reducing SA.