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accce - Over the years I have gathered a lot of data as to why the OEM approach design a certain way. There is so much misinformation on the internet floating around on forums that many companies get away with false advertising. Because most of the data are classified, they are not openly shared.
For example, if a company produces a rear exit diffuser and measures the coefficient of lift and drag inside a controlled wind tunnel. That data is only an estimate. The problem with wind tunnel data is that any speed over 60mph begin to elevate the temperature of the air as it is recycled back to the front. This is referring to a full scale wind tunnel. Then you have the missing variables of real world scenarios such as turbulence from the spinning wheel and drag at the wheel well. Then there is turbulent air from cars in front of you and cross winds. Even if you were to get all of that data down.
If you change the wheels or change the height of the car even by 1", all that data you got is now inaccurate. Even the tread pattern will affect the path of air flow.
Now if you added a new front splitter, that will change the figures of the rear diffuser. Many people out there don't know that if you increase the negative pressure from the rear, it will increase the positive pressure in the front. The best practice for aerodynamic design is a balanced approach.
With the M3/M4, I assume the weight distribution is F50/R50. So adding a big GT spoiler to the rear of the car will increase the frontal lift and create a situation where the car becomes unstable at high speeds. You can feel the car get slightly understeer especially accelerating around a high speed corner. Now imagine going through the corner where the apex is cresting over a slight hill. Even your brakes will be out of balance in this situation.
So any figures a company provide is not really going to apply to you unless you kept your car at factory spec. The only figure I would pay attention to is the reduced drag coefficient, as that can help more for weekend track warriors. Unless you have power to spare, having additional downforce will not improve your track times. You are just going to make your car work harder.
I know a lot of OE has been turning to CFD but even has its limitations. In my opinion the issue is that the predicted figures never match with real world simulations. CFD is used to help designer and engineers to determine if a new approach/concept is worth investigating.
Hope this helps.
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