the midpipe is the problem with the stock system . it is a restrictive crossover.
no matter what you see or hear slip ons are just cosmetic because they use the same midpipe. 4/1 pipes allow you to exhaust more spent fuel.
to realize those gains you will need a power commander to load a new fuel map. either get a fuel map from your pipe/power commander supplier or go to a tuner on a dyno.