4-2-1 design gives more low end power gains at the expense of higher RPM gains. 4-1 design does the opposite giving higher RPM gains at the expense of lower RPM gains. Its the same way with exhaust systems Axle back are good for torque and low mid power. Cat backs are good for higher Revs and mid/high power
SS headers are better than CC headers cuz of construction mostly. the SS ones will take more abuse (especially if u have a lowered car) and they dont crack in the colder months