It's not any more or less difficult to put a cat and O2 sensors in the exhaust. Maybe a little more money if you do custom exhaust with a new cat. The ECM needs O2 sensor input and it monitors the charcoal canister evap system and the EGR system. This might could be defeated via resistors, but I'm not certain. My EGR solenoid is bypassed via a resistor and it hasn't given me a CEL since the mod. Evap system on the other hand.....