I had, 5" hi flow cat, 2x2.5" cat back exhaust, pod hanging off the turbo and didnt get a engine light, and the car ran fine. I did get it tuned after although and the tune made a huge difference. If you get a engine light from a hi flow cat its probably because the heatsheild over the dump pipe has been bumped when the Cat was installed and is touching the wires on the oxy sensor. just make sure your boost regulator is clean and on firmly to the pipe to the turbo and it shouldnt overboost oh and oil. nulon 5w-40 full synthetic 6.6Ls worth ryco oilfilter will do for $10- and I got RDA slotted for the front $160ish from the keysborough store. no warps and still pull up hard after 20,000kms if the spark plugs havent been changed in 80,000+ kms , you may wannaa do that too? and maybe replace the diff oil