MAF based tuning logic (directly measures incoming air) cars are very sensitive to air intake changes which is why a tune is required when you modify the intake. However, many modern cars like the BMWs we support or the Focus ST and Fiesta ST use speed density (calculates incoming air via other sensor input) tuning logic. Therefore, you can make modifications to the car without effecting the tune. However, it is torque based tuning logic, so modifications without tuning won't make a very big impact on power since the car will by trying to meet the same torque targets even with free flowing parts installed. We have done some dyno testing and tuning with a wide variety of parts installed and haven't seen any issues. And before you ask, I unfortunately don't have dyno plots to share just yet 
Ian
Ian