Parallel axis gears, such as transmissions, rob far less power than perpendicular axis axis gears, such as rear ends and steering boxes. The lack of a transmission is beneficial in terms of overall car weight and driveline packaging. Getting rid of the clutch and flywheel gets rid of rotating weight which is a big advantage in accelerating and slowing down the driveline, but doesn't have a big influence on steady state power. Driveshaft alignment will also have an effect on rear wheel power, and any difference in tire circumference will show up as a power loss. The general rule of thumb I learned is that any gear set that changes the direction of power (ie rear end, steering box) is going to cost you 15%. Then add on losses from there.