i'm actually building a frankenberries exhaust (frankenstein/scare the dingleberries off ya) for my speed6 switching back to single pipe/single exit. the exhaust housings alone will keep it stock looking/sleeper but the one 3inch pipe and straight through muffler will definitely be a hp improvement if only 20hp.
the exhaust consists of 3 inch Corksport Resonated Test Pipe --> CP-E 3inch Catback Straight Pipe --> single 3inch straight through Magnaflow muffler. The sound should be quite perfect IMO cant wait to install it. waiting on a Corksport Urethane Exhaust Hangers group buy in the Speed6 section. if you have an exhaust that is bumpin around this is a great solution!
also you have to consider that car companies make dual exhausts for 4 cyl cars because they make less noise, vibration, and drone! they all do dB tests inside the car at idle, driving/cruising , WOT, and if they consider the exhaust noise to be too roudy for the average joe, they'll design a way to have resonators/mufflers added to gain approval. almost everything restrictive done to the speed3/6 was because of noise/vibration.