...the rear O2 sensor is there just to check cat efficiency. Before committing it to fact that a MIL will not be set by gutting the cat, try this: run it at wide open throttle (WOT) for some period of time after the car had a chance to warm up. A couple of minutes if you can. The A/F will revert to open loop, and there will be some WOT A/F enrichment. Car makers run pig rich at wide open, as that's when more power is needed, more raw fuel is necessary for cooling combustion chambers and the tops of pistons via quenching, and where fuel economy and emissions homologation is never tested and certified. At that condition, there will be more raw hydrocarbons in the exhaust, blowing right through the hollow where the honeycomb used to be. And, my guess is that your rear O2 sensor will kick off a signal voltage which will light a MIL. I would be curious to know what happens when you try this. If it doesn't set off the MIL, then I would conclude that the OBD cal in the ECU is way off. Or, maybe this was somehow manipulated by Upsolute when you had your chip done?