If increasing the horsepower or torque of the engine on a Volkswagen or Audi vehicle newer than 2006 that has a DSG transmission, yes the transmission will need to be tuned.
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.
Upgrading the exhaust does require reprogramming the engine computer. With the tuning software that we use, we guarantee no check engine lights after reprogramming.
Most passenger diesel vehicles from model year 2007 and newer do have diesel particulate filters or DPF on them. Volkswagen and Audi diesel cars from 2009 and newer all have diesel particulate filters.