Just what the roadster needs

[vmax1974]

should have a small effect on the road but the real benefit will always be on track and on motorways and as spud said the rear panel will not act as an airbrake to slow the car down other things you could do is some vents in the top rear of the rear arches to unload the pressure build up in them that will reduce the airbrake effect too so should also help with fuel costs