re: Anyone use a neoprene wet suit top for cool water conditions?

A 3/2 is all you need for Louisiana. I'm not really sure what your intended use for the suit is, but wetsuits are not designed to keep water off of your body. They're designed to do exactly the opposite. Wetsuits keep a thin, insulated film of water around your body that stays heated by virtue of your body heat.

That said, you want a suit to fit snug, but not overly tight such that it's impossible to get on and off. Especially if you're doing some type of physical activity. Paddling a surfboard in a 5/4 is a lot more difficult than paddling in a 3/2. 1.5mm isn't good for much other than keeping the wind off IMO, but a 3/2 will get you through pretty much all but the 2-3 coldest days of winter and still give you some flexibility.

As far as which type of wetsuit to go with, it would help to know what you intend to do with it. If you're kayak fishing, I'd probably go with a longsleeve springsuit. A sleeveless suit would be much easier to paddle with but you will lose warmth in the shoulder area.