I have a high flow cat on mine, and IMO they're not worth the money if there's nothing wrong with your current cat. When I bought my car the cat was stuffed, and it was only about an extra $50 for a high flow instead of the standard one.
I've been told that ford I6 cats flow very well, but another option you could consider is to try one from a different model (i.e. 5.0 V8s, even Gen3's). The advantage of this is that almost any cat from any car can be attached to your existing exhaust system and it may work out cheaper than getting a new high flow one.
Lastly, if you're gonna gonna hollow out your cat, then get someone to weld a piece of straight pipe inside it, as it helps flow and significatly deepens the note. Having said this, I DO NOT recommend that you hollow out your cat, as if you are caught i think there is a minimum $5000 fine, not to mention the fact that the fumes from the cat-less exhaust will kill the environment (and people!).