This is undoubtably the most popular OEM+ upgrades for this platform. Pure bolt on for any MK7 GTI (does not fit 7.5 without modification to the tips).
Replace the horrid sounding factory muffler with this free flowing Clubsport S unit. Piping matches the diameter of the GTI perfectly. Can be fitted using a clamp which is recommend to purchase together.
https://track-hack-parts.myshopify.com/products/gti-clubsport-s-muffler-clamp
ETA 3 weeks