All the old shaftdrive Yamahas and Suzukis had transverse engines. No need to"turn" the engine, and powerlosses are not that much with these sort of outputs.
Yamaha's V-max is an example.
Quite true. It is a fallacy that a shaft drive loses a lot more power than a chain. The latest shaft drives are close to a chain in efficiency and actually better than a worn chain and sprockets. If a shaft lost as much power as claimed, the heat it generated would cause it to melt.
The real drawbacks of a shaft are increased unsprung mass, which affects handling and suspension reaction and also adds complexity. A very powerful, lightweight motorcycle with a shaft drive also suffers from driveshaft jacking, i.e. the bike rises and lowers on the suspension under acceleration and braking. This is the reason why Laverda abandoned their V6 shaft drive race bike project.