I am not sure what actual difference there is between the hi-flow and regular ones as far as performance goes, however there is a difference when you look through them. New regular cats when you look through them you can see blurry light/images about the size of a dime at the other end. With my magnaflow hi-flow cat the images were clear and were about the size of a quarter. The other difference is about $50 more for the hi-flow one.

You don't want to open up the exhaust too much w/these engines, unless you are planning to operate above 4k all of the time, you need some amount of back pressure for low end torque.