well, my two cents (probably being worth as much):
not really sure if auto-x veterans make better road racers than road race veterans make soloists, never seen any concrete evidence, but assuming (for the sake of arguement) that is the case, i would attribute it to the following.
Auto-x offers (in an average scenario) an opportunity to learn and use much more extreme car control techniques much more frequently. when I say extreme i mean greater steering angles, throttle transitions, more frequent direction changes, gear changes, etc. a soloist who can proportionately apply their skills on a track may have an advantage over a driver who drives exclusively on road courses. however that proper transition itself requires a great deal of talent.
i think its more likely however that road racers and auto-x drivers have different expectations of performance driving, auto-x tends to have more of a practice stigma and track events seem more like a release.