How much power do high flow cats give you. I am getting headers with a cat back should I buy high flow cats or us the shock ones on my 1998 z28 m6

 

Installuniversity.com has a flow comparisson of cats. There is a huge diff.
HRM had a dyno comparisson of hiflo cat vs no cat several years ago and there was only a 2hp loss from the cats. So pretty much take the gain from !cats and subtract 2 hp. I've heard a 10 hp gain from !cats.
If you're looking to buy some cats. Ebay has some carsound/magnaflow (as shown in the above mentioned flow comparisson) cats for under $60 ea. shipped.