There are really a difference in performance, power or torque, between a original catalytic and a high-flow catalytic converter or vacuum on a civic SI if I do not change the remainder of the exhaust (I just changed the muffler, ECU, cams ITR and Iceman Like)

 

High flow cat is more beneficial when your engine is flowing a significant amount of air more than stock, so if you keep a stock exhaust, you shouldn't lose power, but you won't gain any either.

 

A high flow is also a good bit cheaper than stock, so if you need to replace the cat anyway you might as well go with a high flow unit. But, you won't see any increase in power unless the rest of the exhaust (header, cat back) is opened up and the engine actually needs the extra flow capacity.