A handlebar bag and other parasitic drag loads on the bike, an upright position on the bike, riding speeds in the sub-20 MPH range...the advantages offered by something like a Mavic Cosmic 50 MM wheelset will be minimalized, but some Watt savings / speed gain will still be present. I don't see any local randyknees types using aero setups at all. Like you said, it's probably the last priority of a touring rider.
For performance riders it is often the first priority. And quantifiably so. The web is easily searched for the data...from college / team wind tunnels and manufacturers testing results to users' data to triathletes finishing results to time trialists' records to road racer's data to age-graded racers' benefits to the performance gains seen by my team mates...aero works.
Your rear disc wheel did provide you with some measure of speed increase / Watt reduction (now exceeded by the better 80 MM, 90 MM and 100 MM wheels in some cases). How much you could take advantage of the disc was, of course, dependent on what your legs could output.
I have to agree that training is THE most important factor in performance...period. After that, aerodynamics, fit / position and equipment weight rank next.
