Yes, any changes with the factory components will void the factory warranty.
No, 3 inches is simply the size that we carry to keep selection simple. 2.5 or 3 inch free-flowing exhaust is more than sufficient for the diesel engines that we work with.
Yes in a straight pipe version, your car will be louder. With a straight pipe into the stock muffler there will be minimal increase in sound with notable turbo whistle.