This is a great thread, have also been thinking about the 11mm vs 13mm subject lately after purchasing an airbalete. Wasn't sure if i should have gone with a 13mm barrel gun.
So 11mm tube is suitable for 7mm shafts or smaller and 13mm barrel is best for 8mm + shafts?
I think in the real world - the difference in overall performance is pretty small - great subject for a forum debate but, I wouldnt worry about it to much.