If you look at any SUV with same weight from same era, you will find much bigger, most likely dual piston calipers. WJ is in the similar weight ballpark of 4K pounds (taking into account our hardtop, winches, heavy front bumper, rear tire carrier bumper and heavy jack - have all this on my rig), and it comes from the factory with dual piston caliper (Ecabano or whatever the Japanese name it has) and larger disk with more contact area between pad and the disk. It has much better brakes. Its dual piston caliper is similar to something that is in the BMB BigBrakeKit.
So, yes, the factory brake system is sufficient, if you don't start to overload it, don't put oversize tires, keep distance from the vehicle in the front, and don't forget to brake with transmission/engine when you go downhill, otherwise you might overheat the brakes easily. It cannot be driven like a modern car w/o better brakes.