Forget the hypothesis about stress, technique, risk, etc, etc.... there's one extraordinarily simple reason why 558 doesn't do a barrel roll, and never will. She's a display aircraft and not an aerobatic one, therefore is not permitted to do anything classed as aerobatics, lest anyone get into serious trouble. If she did go all the way round, the paperwork would probably make the collection engineering documentation look like light reading, and there'd probably be an immediate end to flying, never mind the person driving probably losing their licence!
I hadn't heard any ideas of the aircraft rolled by Falk at Farnborough being damaged by it, but the story goes that he was told very definitely never to do it again even though it was apparently agreed at the time is was safe to do and posed no risk to the airframe or the crowd. However, it was allegedly worrying the people on the ground (maybe it was an excuse or cover-up?). I presume it wasn't unheard of to roll production Vulcans as I'm sure somewhere I saw comments from ex flight crew suggesting the B1 marks rolled more easily than the B2. Oh, and Mike Bannister and his French equivalent rolled Concorde just because they could....
