Has anyone else noted a drastic reduction in braking performance with Zipp's NSW Showstopper braking system as they age? Right now the dry stopping isn't great, but wet stopping is practically nonexistent. The brake pads are the recommended ones and they are adjusted correctly. The brake track looks and feels polished. Zipp/SRAM examined the wheels and said everything is fine. This is the first time that they have heard of this problem? Really? It used to be that I had to clean off copious amounts of brake dust after each ride - not any more. The warranty will be up shortly. Does anyone know of an alternate Shimano style pad that can be used without compromising the wheel?
The showstopper braketrack is a textured finish, with very shallow grooves carved into the surface to make it rough and abrasive. This abrasion is what was grinding the powder off your brake pads originally. Unfortunately the rubber of the brake pads has also been very slowly wearing down the surface of the brake track as you've used the wheels and polishing them to a smoother finish.
To test if this is causing the issue, you would need a second new set of wheels with the same braking surface. You will be able to feel the difference by simply dragging your nail along the brake tracks to see if one is rougher than the other.
As for improving the quality of your braking with the current wheels, you simply need to look at which brake pads perform best on carbon wheels with a traditional smooth brake track. Personally, I always found that the Swissstop Black Prince brake pads worked best for me. In the past few years, I've used carbon wheels by Syncros, Roval, Fulcrum, Campagnolo and Shimano Dura Ace, all of which had a smooth carbon finish. Regardless of manufacturer, I always found that the Swissstop pads worked better in any conditions than the pads supplied by the manufacturer (except the Rovals were supplied with Swissstop pads).