Additional aero always reduces your top speed. It creates two distinct forms of drag, skin friction drag, due to added surface area, and induced drag, which is drag due to lift, or in this case downforce. The effect gets more pronounced with more downforce, there really is not set number. It’s like asking if fat people in your car make it slower, and how fast can I go before I have to make them diet.
The front diffuser isn’t really a diffuser per se, it’s just a splitter, so it’s at a zero or close to zero angle of attack. The diffuser in the rear is meant to channel the air out at an angle of attack, while the front splitter just builds a bit of extra pressure. In open wheel motor sports, balancing is a bit easier, because they can just run a front wing, but in regular kinda cars, the splitter will have to make do.
Not sure what you mean by that drawing… if you’re looking at the car from the side, and it’s like
F ________ _________ R
… /
Then you sure as hell don’t want anyting like that
if however you’re looking at it from the front or back, and it’s like
L__________________R
…|…|
then you do want those, because they help channel more of the flow through the middle, so it doesn’t get disturbed by all the stuff coming from the sides.
Edit: dots have been added to put stuff in it’s correct place, otherwise forum moves it to the front