The primary purpose of an aftermarket downpipe is to remove or replace the stock
catalytic converter with a better flowing unit. It also increases the exhaust
diameter for better flow. HP gain is 15-20HP. This figure is highly
debated as different manufacturers use different dynos with different cars with
different levels of mods. It also varies because some downpipes use one high
flow cat, while others are catless. The addition of a larger diameter downpipe
equates to more boost pressure.
You'll be getting one of the nicest downpipes made, with beautiful machining on the front and back flanges, fully polished 3" stainless steel pipe, and smooth mandrel bends. Design to free up tohe exhaust flow by removing the main catalytic converter. With the increased flow, the turbo will respond and spool up quicker. Whenever you modify your factory catalytic converter, you may end up with a "catalytic converter efficiency" check engine light (CEL) code. Although you are less likely to have this code if you do have some kind of cat in the system, you may still get the code, even with a high-flow cat.