it can't hurt-you are still freeing up airflow by replacing the narrow stock unit with a larger diameter one. It won't be as much of a gain as with a high-flow cat or race header, but I know some here who have added it to thier stock cat with a little gain.
i added a bosal flex to my exhaust and it definitely made a difference, especially for the price. the stock flex added as much restriction as the whole system after it from the butt-dyno. i still have the stock header/cat, but for $130 it was a great addition.
when I did my Bosal cat-back, I added the flex pipe on as well, since it couldn't possibly hurt to further open up the exhaust system... no idea if that alone added anything to my driving experience, but as a whole, the car is more fun after the mods... can anyone guess what my next exhaust purchase is going to be?