Surely you would only put a rear aerofoil/spoiler on the car if there was a traction problem at high speed, or oversteer cornering at high speed.
I have never been able to un-stick the rear, as there is not enough power, and very good traction.
Any aerofoil/spoiler would reduce top speed (hence the DRS on F1 cars).
The front spoiler was added to reduce understeer, as the front is very light. I have had understeer when using a TCS on an airfield in competition (sprint at North Weald, Essex, 1979 using 75032429P).
IMHO, the rear spoiler is just for boy racers, to try to look good, and is not required to improve handling.
Alex in Norfolk.