diffuser

Would you gain a lot from Rear bumper diffuser on a car that is used in road racing like auto x, circuits, touges, ects ? Or do you gain more from one in a drag car?

I've been looking for some aero bits for my civic build and unfortunately there really aren't any functional pieces for our cars. So...I'm making my own. I made a tracing of the bumper and then extended the area slightly all around till I came up with an overall shape that I liked. I just...