definite efficiency loss in shaft drive, atleast if its set up like it is on cars and motorcycles. since its not direct drive from the pedal to the rear well you lose efficiency. with a belt it atleast be direct drive, the efficiency loss would occur with belt slippage.
i suppose a shaft drive that was set up similar to old steam locomotives would be direct drive, but im not sure how it could be done, i also imagine peddling might feel very strange also. and no idea if its effecient or not, be a neat experiment though.
wish i had a full service machine shop in my basement to try this stuff out
o a full service machine shop, and a ton more time and ability