Those numbers are very impressive. If you figure 20% drivetrain loss (which I think may actually be a low estimate for a Subaru) the '09 WRX make 290hp/340tq at the flywheel!
As for the linearity of power and torque loss, it has to be a linear loss. Horsepower is simply a function of torque. Torque is the true measure of the energy the engine/car is producing. To arrive at horsepower (as used in the U.S.) you simply take torque*RPM/5252.