i dont want to speak beyond what i know myself.
my experience is limited to an h22a accord with a stock accord cat, later switched to a test pipe.
and an h22a civic that was open header switching to a full cat back with high flow carsound cat.

the h22a accord felt like it was choking at high rpm, the switch to a test pipe got me better high end power as well as a lot more buzziness to my exhaust. still pretty quiet though (apxi dunk)

the h22a civic...open header was weak on the bottom end, screamed up top. i put the exhaust and cat on....bottom end felt a LOT better. high end felt unaffected too.

speaking from others knoweldge....i ahve been told as much as 5-7 hp from a test pipe or high flow cat. you should feel it a LITTLE by the butt.

The way I see it a test pipe is $40 or more. It doesnt sound as well, the car isnt designed for it, and there is always the risk of getting caught.
with a test pipe and getting fined.

with a cat you have nearly the same power gains, the car is designed for operation with a cat, and you are at no risk for fines....cost is $60 and up...