I use Ravin mufflers. Now, admittedly, I carry a V8 engine, but at 279ci, it's a smallish 8. The performance difference is noticeable, thing had a bit more life throughout the powerband. According to my dyno, I theoretically may have gotten about 10 horse and 15 torque peak increase, but that's a rough estimation. I don't have a pre-ravin dyno. I only got the mufflers, didn't want to go extremely loud, after all, it's a Cadillac. When idling, there's a quiet, subdued rumble, but when I rev it up, It sounds very sweet. Nice and deep, similar to the tone (note tone, not the sound type) of a Flow'd Camaro with the 5.7 My sound is smoother because of the valve difference, but the deep rumble matches the deep of a Camaro with an extra liter of displacement. I don't know if you're llooking for pure performance, so I don't know of the Ravin is the ultimate speed increaser, but as for sound effect, I haven't heard anything with the same deep rumble effect on an equally sized motor.
