No, a diffuser, like the one in the pic, uses the venturi effect to re-introduce the air under the car into the main airstream without causing as much turbulence. It's not just a rear undertray it's more than that. it has vertical fins sticking out to direct the air like the vortex generators people put on top of their car. I think it would make a difference in any car's aerodynamics. (that or it only helps at speeds above 200...)
Best fill up so far is now 29 MPG!