Modern cats, especially good aftermarket ones, flow well and impede peak power quite little. Actually, the power difference between no cat and a clean one have been shown to be several at most. For several more horsepower, you also get an everlasting CEL and the vehicle becomes technically illegal in most states (whether or not emissions are actually ENFORCED is another matter).
Cats also make decent mufflers with the right exhaust setup. Me, I'm running a Magnaflow unit with no muffler and the truck is nice and loud but not ridiculously so. I mean, I've driven past many cops with this setup and they've never bothered me. Hell, the few trucks around here that still run twin straight pipes are a lot louder. So, basically, if you lop off the muffler and run a cat, you still yield improved exhaust flow without making things too loud AND you don't have to worry about the other crap that results from not having one.