Can anyone tell me what exactly "High Flow Cats" do? What does it do to the stock exhaust sound? Does it make it louder or higher pitched? I really have no idea. I appreciate whatever you can tell me. Thank you.

exactly what it says. higher flow than the stock cats. stock cat is very very restrictive. same concept as having aftermarket exhaust.

I've heard that some of the cat mods can cause a slight raspy sound in the exhaust.
Other than that I would expect a little increase in overall sound, but not much.
C.

get some testpipes. that would surely change the sound and leaks.