I would say that the gains would be very similar. I know that my stock exhaust on my denali was very free flowing and from what I can tell the ss and the sierra denali have the exact same exhaust setup. I personally would have to say that gains vs. magnaflow would be about the same, gains vs. Flowmaster ( I think the stock would be better then flowmaster) However flowmasters sound good( exept for resonating in the cab) I love the sound of my magnaflow, Its not quite as loud as i want but it sounds great.