To be honest faffing around with the manifold into the CAT pipe and then running out of the CAT pipe into 2 seperate pipes looks like a waste of time to me. The 4:2:1 manifold employed as standard on our cars, is probably the best compromise for use on a road car - it gives good torque properties and not too much problem with loss of power at high revs.
You could change the manifold for a 4:1 type header, but you then have to calculate the lenghts of the pipes precisely to give a good compromise between low/mid range torque and top end power... In my opinion a waste of time on a road car which is not always thrashing away - fine on a rally car though ;-)))))
Best bet is to stick with what you have as far as the manifold is concerned, you have lost the biggest restrictor already - the CAT, then, use a slightly larger bore of pipe for the system from where the cat bolts on back - say 20mm bigger diameter - if you go too big you could actually loose power....