Removal of TJ cat

My Jeep dealer told me not to remove my TJ cat, because the engine may run lean and hot then. My cat get's destroyed easy when I take my TJ in the deep water. For testing I removed the rear lamda sensor and the engine still runs at normal temperatures.

I don't care about beeing street legal, just worried about my engine.

The catylitic converters sole purpose is to convert NO3 to NO2. Now if there is an after cat sensor your computer may interpret the the engine as running incorectly if the cat is missing. The compensating effects of the computer, taken to correct the imaginary problem, could be harmful to your engine. Several racing companies sell "simulators" which merely repalce the sensor with a small electronic chip that tells the computer that everything is as it should be. Finding a simulator for a jeep may be difficult, I know caspers electronics sells them for GM trucks. They are mostly made for racing, cats are the first thing to go on the 1/4mile, so there may not be a market for jeeps....yet.