Performance will be the same if not worse from a 40 series, Ive heard over and over the only things 40's do is give you a loud exhaust... I do have to say I haven't heard a performance single exhaust on a v6 stang yet though. Im sure it would sound good, minimal power gains in my opinion but you never know!
98' 3.8 AODE
Won't ever be satisfied with power, but the car looks soooo nice!