They are called restrictors, they limit the air intake. A larger opening means more air and that means more fuel is needed, ofcourse that will give more power.
A smaller one does not give that less power but it can be noticeable and can save you a fuelstop in a 5 minute qualification. In this situation you have to decide whats faster.
Just start with the largest one.
On the edge there is a number, the black is 9mm and the red must be 7 or 8mm
Speed is only fiction but a 130+km/h MRS4 is for real.