FWIW, i had one on my V6 right after the Y-pipe (where our 3rd cat would be), so fairly close to the engine. dyno showed +5whp with the cutout open (same day, run 1 vs run 2)
it was a cheap one (paid $120) and the valve was leaking at high rpms when letting off the gas, giving it a nasty high-pitch noise, so i took it off...
anyhoo, it was INSANELY loud... i couldnt stand it for more than 10 minutes on the highway... and in town i was too scared of the cops pulling me over for excessive noise, so overall i barely used it !!
they do sound a lot better on V8's too