For $5,000, I certainly hope that mechanic is providing lube. You're going to need it.
I have not replaced the cats on either of our Jeeps, but I use High Flow cats on my Vette. They cost a few hundred dollars. Welding them in certainly isn't $4,500+ worth of labor.
If your cat(s) are failing, you will have a loss of power, but you may not notice it. It's a gradual thing, generally, and your carefully calibrated Butt-O-Meter will not detect it until it's really quite bad. If you track your mileage, you will notice it going down as well.
A cat failing is actually a fairly rare occurrence. They're usually destroyed by something else. If you don't find that something else, you may be replacing the cat again.
There are 3 main ways the cat can fail and the way it failed can help you find the root cause.
Look at the cat. If there's signs of an impact from road (or trail) debris, then the substrate may have cracked. Once it's out, you can usually shake it like a birthday gift and listen to the rattle. I won't suggest fixing this problem by staying out of the rocks. Just add some armor.
If there's no damage, cut it open. If the substrate is melted, then the cat is overheating. This is usually caused by an engine running rich. Suspects would be poor compression, misfire, injector problems, faulty O2 sensors...
If the substrate is plugged, then you're looking at things like head gaskets, burning oil, or running E85/Diesel/somesuch in the Jeep.
