I've got the Sig turbo-back. Highly recommended.
It's slightly louder than stock and is clearly deeper in tone. I drove it from Dyno4mance/Georgia to Cincinnati and it was not fatiguing, a bit like the passenger cabin in a learjet in tone, but quieter. For my four cents, go the turbo-back in one step. The catback doesn't give much in performance, and staging it to a subsequent turboback can have fit problems depending on the version.
Doug in Cincinnati