The ultimate sound varies depending on what your backing it up against. However, we ONLY run XR1 on our builds. Our shop Camaro actually picked up .12 ET by adding them compared to none at all (open headers).
Most of our client builds are Big Blocks and they sound super aggressive. If you are annoyed by interior resonance then these are NOT for you. They are free flowing and loud.
Here is a 510ci BBC with 3.5" XR1's we built a few years ago.
https://www.youtube.com/watch?v=DHX7xXqHSpc