"Pre-cats" or "light off cats" as they are sometimes called typically reduce warmup emissions, but once the engine achieves operating temperature and closed loop operation, they generally have little or no effect, so field emission test results are not likely to be significantly affected with or without them.

That being said, it is illegal to modify the original emission control configuration. The FBI probably won't come to snoop around in your garage, but commercial shops have to be careful about removing or modifying catalysts.

Duke