A few things.
1) Resonators do nothing for performance, they are just a "sound part" of the exhaust. Gets rid of resonance or drone in the cabin during cruising. Keep it in there if you like your car to sound good.
2) Low Catalyst means the cat is most likely bad. Cutting it and the U-Bend out NOW would be a great upgrade for your car.
3) Don't go by the DIC. It isn't very accurate. Go fill up your car and run it down to hardly anything. Fill it back and see how much gas you put in it. Then figure your mpg's that way but taking how many miles you drove divided by the gallons of gas you put in. WAY more accurate, even though it is old school it works better than a computer IMO.
