Originally Posted by some bs internet article

However, if you have a stock engine with no modifications, and relatively new OEM catalytic converters, you won't see much of performance increase going with high flow cats. The catalytic converters on a stock vehicle (especially a newer stock vehicle) just aren't that restrictive until there have been significant performance upgrades.

Put more simply: If you've made major upgrades to your engine, which are defined as increasing power 20-30% (or more), then high flow cats are for you. If you've just added a cold air intake or performance engine tune, your money is probably better spent elsewhere.