I'm glad that you enjoy the Avistar, it is a great airplane & it is a ton of fun. However, in your friend's case, having a flat-bottomed airfoil has no effect on the ability of his model to do rolls. If he wants more agility, get him to increase the throw of his ailerons, & also move his C-of-G back a little. Aircraft with flat-bottomed airfoils (trainers included) can fly pretty much everything in the book --it just takes a good set-up & suitable pilot skill. WWII fighters had flat-bottomed airfoils, & agility was their business.