None of these replies has helped him in any way. It's his car, let him do whatever he wants to it. The blinky light gauge isn't going to give him an exact air fuel ratio, but it can easily tell him that something is wrong as it's sitting at dead lean at idle instead of cycling like it should.

Maybe the exhaust shop messed up the o2 sensor and it's messing with your fuel trims? That backpressure thing is crap- if you've got a full exhaust w/ test pipe, you shouldn't be having any problems. Running open header, yes, it might throw things out of wack somehow, but you don't need a cat choking up your engine for it to run.

*edit* Have you checked for exhaust leaks? Maybe you have a big exhaust leak at the header flange? That can mess things up.