The only way Mazda can void your warranty is if the dealership states the mods that you have, then it's up to National Mazda if they want to void your powertrain warranty or not.
Actually, according to the Magnuson Moss Act, the onus is on Mazda to PROVE that your modification directly attributed to the malfunction in question. There must be hard evidence that they can point to to prove this. They cannot take a picture of a mod on your car and say "historically X mod has caused Y malfunction". They must PROVE that your mod caused your malfunction. It must be evidence unique to your vehicle. Without such evidence they are only blowing smoke and are in violation of the Magnuson Moss Act.