With modern cars this isn't a problem even with an N/A as the cat is providing enough back pressure. With a turbo it'd be better to have a straight through exhaust with no CAT of the correct diameter for maximum power/torque (go to large & the exhaust gasses won't move fast enough at low rpm to get the full benefit)One of my family friends gave his opinion saying he thought it was a waste of money at 400 pounds fitted and i'll probs lose power and low end torque.
On a slight deviation, has anyone had experience with SuperSprint exhausts? I've had 2 full manifold to back box SS exhausts on different cars & both of them has a subtle quiet but sporty sound & gave good power gains.