uhhh.....a hi-flow cat isnt going to pin you into your seat when you step on the peddle..
Its just a free flowing exhaust.
Take two straws.
One from McDonalds and one from Burger King.

McDonalds = stock cat
B.K Hi Flow.

blow air through them both and you'll notice the Burger king ones flow better cause its a bit bigger and less stuff in it to restrict air flow.
Just know your car exhaust will smell becuase of that.