Originally Posted by kitw
The challenge street will pass the tail pipe and OBD-2 tests, but won't pass visual since it eliminates the secondary cats. (most places won't catch this)
None of them will produce power without removing the primary cats, though. You might as well do a Challenge Sport, Turner test pipes or Innotech if you don't mind some rasp. The Gintani/MXP/Fabspeed that put HFCs in the same position as the stock cats won't make power like the the ones that move the cats downstream.
Our street X pipe showed gains across the entire power band actually, even with keeping the stock primary cats.
It has been verified by some of our customers dynos as well.