It doesnt matter how you go about achieving the emissions result, as long as you do. Or can get a bent ticket. If you need a cat then put it back on. Valvers have been known to pass an emissions test with an ecotek on and no cat. Even without an ecotek or any other kind of fiddling about, a clean, well looked after engine should be there or thereabouts anyway, the cat just makes sure it is well into the legal emissions band. Obviously newer engines are likely to naturally make less emissions which is why you cant really compare a saxo to an old valver, although they are both subject to the same regulations.