Roof spoiler effectiveness

[Chris Percival]

I don't think a lip spoiler is that clever in our application. They just detach the laminar flow from the rear of the car, thereby eliminating any lift that flow might be generating, nothing more..

[Turbo Haraune2]

Well I am a helicopter engineer, and I say you cannot use them in a combination.
Just as individual spoilers, because the roof spoiler wil destroy the airflow to the tail, and you loose all effect of it.

My opinion based on experience is that the Tail spoiler gives you most downforce and makes the car more stable. bigger=better!!