little adjustment to the comment about the cat:

the cat works best NEAR stoich.... ideally, you would run the engine at stoich, then every once in a while(by this, i mean a pretty short term) send it slightly lean, since the cat NEEDS free oxygen to do some of it's work. alternatively, you could jump pump AIR directly to the cat at all times(until it gets too hot), that will keep your O2 sensor happy(and accurate) while keeping the cat supplied with the components it needs to catalyze the exhaust gasses.

the compromise to this is the path most automakers took and send the engine slightly rich and slightly lean alternatively using the narrowband for feedback. it's not a perfect system, but it does work pretty well.