Adding a cat is not that bad of an idea. It will greatly reduce NOx (oxides of nitrogen) which definately has a stink to it. Air injecton makes them even better, but not really necessary if you install it close to the manifold, and keep the tune up spot on. Some type of shielding from the floor is a good thing. Modern cats will have a very negligible effect on horsepower if any. Some pretty darn fast, quick, and clean cars have them.