No worries. According to Cobb, the rear O2 sensor is responsible for up to 20% of fuel corrections; using the defouler could negatively impact MPGs. However, without the defouler you would get an engine code for a bad catalytic converter which could also negatively affect MPGs. The remedy is a tune that basically disables the code. I didn't get any codes with the catted pipe. My tune I'm currently on has the code disabled so I have no use for the defouler. I made the swap because I want the most power available; spools a little bit quicker; and of course it sounds more awesome. It's only about a 5-10hp difference though. It was suggested by my tuner to switch over, so I did. Pros of the catted pipe are mostly relevant if your car is registered in a state with emissions testing, or if you just generally car about the environment. Cons of non-cat are the smell; reeks of fuel but I like the smell.