Downpipes on the MK7 GTI: Further Down the Tuning Hole
Outside of an ECU upgrade, a downpipe is probably the single best performance upgrade you can do on your MK7 GTI. Aftermarket downpipes accomplish this via two changes; Aftermarket downpipes are almost always wider in diameter than stock, in addition aftermarket downpipes will utilize a more efficient catalytic converter, if any at all. This allows exhaust gases to flow out from the turbo at a greater speed. The ability to move more exhaust gases, and do so faster allows the turbo to spin more quickly, decreasing turbo lag (the time it takes the turbo to spool up). You can expect an aftermarket downpipe to net you between 15-20 max horsepower and torque (without a tune), in addition to the increased responsiveness of the turbo. Aftermarket downpipes can also aid in reducing the temperature of your engine compartment, allowing cooler and thus denser air to enter the engine. Cooler air going into the engine equates to better combustion.
To fully take advantage of a downpipe on your MK7 GTI, an ECU upgrade is required. These software tweaks will enable your engine to take full advantage of the huge restriction you just removed between your turbo and cat-back exhaust. Tuning will also help remove any associated CEL (Check Engine Light) caused by an alteration of emissions components (catalytic converters) when replacing your stock downpipe. Generally downpipes will not alter the sound of your stock exhaust dramatically. You will get a slightly deeper exhaust note upon idle and acceleration, an increase in sound output is to be expected, but it is not as drastic as adding a cat-back exhaust (the portion of the exhaust that follows after your downpipe). Most downpipes are designed to mate up to your existing stock cat-back and thus do not require replacing your entire exhaust system or any further modifications.
Reminder: Make sure to verify that your new downpipe will conform with your local emissions standards/requirements. Some downpipes like the Unitronic MK7 GTI replace the stock catalytic converters with a higher flowing unit, helping you keep your car in compliance. As mentioned earlier, replacing your downpipe may or may not trigger a CEL from your vehicle, this can be removed via a tune or an oxygen sensor spacer.
Downpipes for MK7 GTI
How to install a MK7 GTI Downpipe