Both on a KA and an SR and even catless, it has been proven there are major gains… however would you live with the noise is the question that’s all.
Edit:
Honestly I got a KA with a BRM exhaust with 2.5" ss piping with a 22" resonator, dual tip muffler and a catco high flow cat… to tell you the truth its pretty damn loud still… its still damn loud… but i think it may be due to the engine, tranny and even exhaust mounts that ive put (all solid basically). I’ll have to double check for any leaks if any however… we havent noticed any (tried to hold cigarette next to exhaust to see how the smoke flows LOL ghetto test but hey…)
Ah well… havent seen my car in months now… its probably very dusty in storage… but yea… its pretty loud. nothing better than stock exhaust if you want quietness. I still love it… its just i dont want attention. Wife hates it, she thinks the car is scary all the time lmao.