The problem with the Blactop Zetec is that it's got a very restrictive manifold with the cat right next to the engine which restricts power and makes it very quiet, fine for a focus but not ideal for a sports car. The silver top has a more conventional set up and produces a bit more power as standard.
Changing the silencer for a less restrictive one will increase noise but won't do much for the power. To achieve that you need a new manifold and cat which is why the Librands system is the price it is.
My last car was a Blacktop and I replaced the standard system with the full Librands setup which does make a noticeable difference.
