My friend was telling me to get a high flow CAT filter but I was wondering if it does add horsepower and how much does it help.

 

It doesn't make a big difference. I have one on my car, (Random Technology) and it wasn't a big improvement. I only got it because the stock Cat was plugged up.

 

It depends on what you have besides that IE: Aftermarket Header and catback exhaust system. If you have a stock exhaust mani, and stock exhaust piping & muffler, a high flow cat wont be all to noticable. But, lets say your running a 2.5 in collector on a RMF header, with a high flow cat, and a 2.5 catback, then its going to make a bigger difference since all the piping is the same size...no bottleneck.