There is all sorts of phyics going on that are being ignored.
You have to look at Static Pressue, Velocity, Volume, a butterfly beating its wings in china... etc.
A tuned system (ala HO Quad4) really can't be improved that much.
I remember readint in Grassroots Motorsports about their project Porsche 944, and how they put a Borla system on it and it was *SMALLER IN DIAMETER* than stock. And yet they had power increases from it.. Can you say velocity?
I would bet that gutting the cat and pre-cats cause some sort of flow problems that reduce velocity and thus reduce power.
