Lave the cat as you will have to refit it every year at mot time. Buy a "sebring" backbox for 230 pounds which is stainless so will never have to be replaced again, bearing in mind that a pattern rusty steel one costs 110 pounds! Forget about the middle two boxes completely, sebring do a long stainless pipe for 40 quid that is just straight through. So all you end up with is the cat and a backbox on your system.
I was worried that my v6 might be a bit too loud for comfort especially on long journeys, but on tick over it sounds like a superbike, and cruising around normally it is smooth and unobtrousive, not too loud at all, but when you give it the beans...oh the beautiful noise!!
Power has improved, drivability has remained unchanged, as has mpg and smoothness. And occasionally on a slowing downchange you get the slightest backfire rumble...sounds good too.
The best quote I ever saw after a review of the 164 QV, was "From now on I am only going to buy cars powered by Alfa V6's"
Mmmmmm The noise, the noise...................