Do I need to weld this to my current exhaust?

No welding required. This catback bolts directly to your factory catalytic converter flange. You'll need basic hand tools and jack stands to get underneath.

Will this fit with a high-flow cat or test pipe?

As long as your upstream system has a standard catalytic converter flange, this system will bolt on. If you're running a non-standard setup, measure the flange before ordering.

What kind of sound level should I expect?

Catback systems add noticeable volume and a deeper tone compared to stock, but it's not extreme since the catalytic converter is still in place. Sound character depends on your engine's current condition and modifications.