A diffuser is basically a pipe, just a flared one, what i was saying is that a diffuser does not consume work, just as the same as a pipe with water flowing through it does not consume work.
Does the diffuser takes this 10hp and turn it to heat energy? motion? electrical?
No moving parts inside the diffuser so no work being done.
To illustrate, a house roof can behave like a diffuser when wind blows over it, but it does not consume power because it doesn't move.
If you increase the system size that considers the whole car and some volume around it, then it's possible to say work is being done by the air to compress the springs and lower the car by a certain distance.
A diffuser is the mechanical device that is designed to control the characteristics of a fluid at the entrance to a thermodynamic open system. Diffusers are used to slow the fluid's velocity and to enhance its mixing into the surrounding fluid. In contrast, a nozzle is often intended to increase the discharge velocity and to direct the flow in one particular direction.
Flow through nozzles and diffusers may or may not be assumed to be adiabatic. Frictional effects may sometimes be important, but usually they are neglected. However, the external work transfer is always assumed to be zero.