Silvan that really is not true. The loss, as measured on a Dynojet or similar transient dyno, is impacted by straight frictional losses (the linear portion), frictional losses (those that are force related such as gears), fluid losses (square of the engine or driveline speed), and inertial losses (driveline, wheel, tire, etc - note this only affects transient dynos like Dynojets). The end result is a loss curve with a constant, a linear and a exponential portion.
For most vehicles in the 200-300 hp range this ends up being around 15% at the peak horsepower speed. As you increase the power the tendency is for the loss percentage to go down. However the actual loss continues to climb and does not stay contant. for a 500 hp car like the Viper a 12% loss would be a reasonable estimate.