Have to agree that's a pretty sweet looking car. Any photos that show the bonnet a little clearer? I think I see a nice looking scoop. If you don't have a picture don't worry about it.
Not to be confused with cut in speed. Below a certain rpm the turbo does nothing as flow through the turbine is insufficient. Once flow (rpm) picks up it can start working. This is often confused with lag. Lag is exactly what ssskiwi says, spool up time.
Sorry to go off track, which Bursons? I'm slowly collecting parts to go efi on my a15 and have been looking around for an electric pump to fill my surge tank. $30 is the best price I've heard of.
At a rough estimate a 100mm vertical sheet of material sticking up from the back will have the same aerodynamic effect as anything else on slowing the bound vortex and hence reducing lift and drag. A wing's only effective if it's out of the wake (above the roof?). You can make it look however you like! At least that's what the fluid mechanics expert at uni taught us. Doesn't hurt to experiment possibly measuring ride height on a smooth road before and after would give an indication.