I cant answer your question about turbofan engines, but my guess is the added drag is negligiable compared to a prop. As far as counter-rotating turbo props, never heard of it, because it does affect efficiancy, how much I really dont know, obviously there are critical engine issues you dont really see in small piston twins that counter-rotate, but like anything else its not a real issue with some training. You'll for sure have to go back to using those rudders on take off though...lol in my best southern drawwwwwl (You may have met P-factor but you aint met his daddy)