All cats are restrictive, they reduce engine efficiency by ~15% in most cases. That's 15% less power or economy (sorry you still can't have both by decatting!
) so will feel more powerful and will allow a turbo engine to develop more boost.
What's often forgotten is that cats clog up with soot and other unburned fuel particles over time, running some Cataclean through the fuel tank combined with a few Italian tune-ups should restore factory performance (in the cat at least) but the ultimate is to decat it and only refit the cat for the MoT.
Usually cats aren't used for silencing, gutting a cat will normally only produce a slight increase in noise and sometimes a tinny rasp as the case resonates, a decat pipe has neither of these problems as it neither acts as a silencer or is the right shape to resonate.
As Mark says, the exhaust system is designed for being quiet and removing the exhaust gases as effectively as possible within the compromise of remaining quiet. However on Volvos, they use relatively large bore pipework in the exhaust sytem so unless you're running a lot more power than standard, changing the exhaust isn't productive - removing the cat would be but that depends whether you want to temporarily refit it each year for the MoT then remove it afterwards and fit your decat pipe.
Alternatively, convert to LPG and legally remove the cat permanently.
