They should not be allowed to fail you for an upgraded cat. In california, where they have CARB, ok, sure. But the rest of the country, you're allowed to replace factory emissions bits with other brands. So as long as it passes the sniffer test and has ANY cat, they should not be allowed to fail.
edit: about about 98% sure on this, so its possible that i'm wrong. And if im wrong please let me know...i like learning new things!