As well, just moving the Cats back is typically not acceptable.

The reason that newer vehicles locate the Cat so close to the exhaust manifold outlet is to expediate it's heat up (or in auto-speak - Lite-off) time, and therefore it can begin to reduce emissions when the engine is still relatively cold.

When the Cat is engineered for this kind of operation, moving it further down the exhaust stream can cause problems as if the Cat does not see enough heat, it cannot burn off the impurities and plug prematurely.

Punx, if he does make the headers for the quad-Cat engines, he probably would be just as well off removing the primary Cats as you had mentioned.

Also, I would be interested if I were to keep the X long term...but I still haven't made that decision yet!