I called and spoke to one of the Magnaflow engineers earlier about their product. He said a 5 to 7 HP increase was about right, enough with a drop in K&N (another 5 HP) filter to be noticeable improvement in acceleration.
I asked about the noise factor, particularly the drone I used to get with the setup I used to have in a prior car I had put flowmasters in. He said the decibal increase from most aftermarket systems is at least 10 db, but with the Magnaflow they are measuring only 2-3 db increase, and with no drone-like vibration. Also, they expect a 1-2 mpg improvement in fuel economy due to the engine not being worked as hard, with an across the board decrease in psi in most driving conditions compared to the stock system. I remember having a slight improvement in mpg in my Mustang from this mod, but it was usually offset by my increased force weight applied from my foot.
