An HKS GT extension kit is part of the HKS exhaust for the Evo. The GT extension kit is made of the turbo elbow (the part that goes onto the manifold) and the downpipe (the part that goes to the cat). From there on, you have the cat and then the Muffler. HKS make a Hyper Muffler and a Super Drager muffler. The Super Drager conforms to Japanese exhaust noise levels (JASMA), the Hyper Muffler does not I believe (?) I have a Hyper Muffler, sounds great, not too loud.