Remove the cat for a highflow cat or resonator, I would not recommend a simple test pipe because most n/a vehicles will sound horrible with a test pipe.

Choose which ever exhaust meets your application & sound requirements.
Some jdm exhaust systems such as t1r are true 2.5 systems, most other systems which are designed for usdm cars are 2.25 due to the fact that they are designed to bolt up to the factory 2.25 cat etc.

Good Luck.