If you are on a tight budget you probably don't need to do the cat. I changed my cat when fitting new extractors and 2.5" system basically as prevention. The existing cat had done 190K and if it sh*t itself then it could screw my new cat back system. As an aside, an exhaust analyser found my new system to be sweet. No CO2 was even registered.
Imo, you dont need a new cat for HP gain, just as a precaution and a cleaner world, if you have the money.
rev@IRC. EB 3.9 auto. VDO head unit, Focal and Sony speakers, tinted, mags, pacemakers, hi-flow cat, 2.5 inch exhaust, K&N, slotted.