An RS Turbo transmission loses around 30bhp. This varies depending on the ambient temperature, the type/temperature of the tyres, the temperature of the gearbox oil (the transmission loss will decrease with back-to-back runs) and whether the Rolling Road operator wants to inflate the Flywheel figure by tweaking the footbrake on the coast-down.

Hope this helps.

IMO this figure shouldn't change as the horsepower increases, because the car still has the same drivetrain, regardless of the horsepower.