I saw the rankings, 4 times within 150 for both account... not really an average, sorry, just stating the fact, no offense, but you are improving with better average lately.
http://www.mygranturismo.net/rankings.php?online_id=cargorat323
http://www.mygranturismo.net/rankings.php?online_id=gtp_cargorat
Looks like you are right about the skill part, a tune could only go so far, I always use my own tune as well, that way I can gauge my own skills easily and improve upon it.
Sometimes when I hit a wall with my own tune ( couldn't get any faster time), I tried to change some settings, then I got better time, then I know my tune have improved. In that way, the tune also limits the driver potential, like a wall that can be pushed further the better the tune is.