go with the high flow cat.
The test pipe is overkill for your set-up.
No engine Needs backpressure,an engine needs balance.
You want to keep velocity up and backpressure down, do that and you'll have good flow. If you have good flow you'll increase performance.
If you go to large on your exhaust piping then you lose velocity, and you end up creating backpressure because the exhaust gasses travel slower, cool down and create backpressure which results in poor flow.
Go to small then you effect flow by creating a restriction which causes backpressure.
The ultimate set-up would have high flow, no backpressure, and high velocity, but you cant have that so you have to sacrifice one for another.