I had an 80 series on Berlinetta #1 for years. I got tired of the interior noise last year and took it out for a Magnaflow single 3" in/out. The car picked up a consistent tenth in the quarter. That's about a 6-10 HP gain.
And, it's quieter in the car now, and still has a nice throaty sound to it.
'82 Berlinetta, 2000 LS1/4L60E. LS6 intake, shaved 317 heads, free-flowing air intake, long tube headers, 3600 stall, 3.73 posi. Best: 12.57 @ 109 at 5800' elevation; 11.82 @ 114 at HPT.
'57 Bel Air, my 1st car. In work: LQ4, L92 heads, LS3 intake, Erson cam; 4L80E, 4000 stall; 9".
'08 GXP grocery getter.
'95 Silverado 2500HD 6.5 TD. Just 335k miles so far.
'96 Vette Base Coupe. New to the herd.