For the intake, @chris_top_her did a test with his OV tuned skyactiv 2.5 using the stock air box & filter vs. stock air box & high flow filter vs. SRI. He found that the car brought in the most air flow (and hence created more power) with the SRI than the stock box with high flow filter, while both flowed better than the stock system on the same tune settings (but with proper MAF sensor calibration). There was a large difference between all 3 (check out the Facebook Mazda 3 Tuning group for the actual numbers). These differences would not likely be as large when running the stock tune.For the exhaust, the tune will bring in more air leading to more exhaust flow. The stock cat-back exhaust on a 3d gen Mazda 3 skyactiv 2.0 is about 2" diameter (not sure about the 2.5), which is good for around 150 hp. The quiet stock muffler is also a bit restricting. With a tune, you will be making more power, and hence wider piping & a high flow muffler would help. Reducing intake & exhaust restrictions will help produce more gains with the tune. More aggressive tune settings can be applied to a car with an aftermarket intake & exhaust.