This is what the code says: "Catalyst system Efficiency Below Threshold malfunctioning or defective catalyst system."

It has nothing to do with flow...high flow cats/test pipes don't throw codes when the O2 sensor is in correctly, and they flow way more than an OEM ITR cat. It's possible that the cat you got is bad...either clogged/poisoned, or the catalyst is loose. If you take the cat off, you should be able to see through it. If you shake it and it rattles, then the catalyst has broken loose and will not function.