On a V8 look looking at about 10 hp increase, if you throw headers in the mix, you could be looking at around 20 to 30 more horses.
Flowmaster is better. Since it's chambered, it will give a little bit of backpressure and increase HP, the magnaflow is a stright-thru and hp numbers will drop.
I myself proved this. With a straight-thru on my Alero, the car ran 15.6 in the 1/4 mile, with a flowmaster 80 and a little bit hotter of a day, it ran 15.2
