2004 Chevrolet Impala gas mileage

They never made a turbocharged Impala. Could you be referring to a supercharger?

You need to be a little more accurate on the MPG you are getting. How far you go on a tankful doesn't tell us anything accurately. We need to know how many miles on how many gallons to calculate MPG. Fill it up, record the mileage. Drive it until 1/4 tank and fill it again. Record the gallons it takes and the mileage driven since last fill up.

Yeah I have an 04 impala ss, sorry about the mix up with supercharger. I've been recording and checking the mileage since I got the converter replaced and at first it seemed to improve to around 16.5 miles per gallon in traffic back down to 13 in town and the same Highway after a couple weeks. 13 mpg and the po420 codes is what led to the vacuum leak fix and cat being replaced initially. Except no check engine light this time. as naive as it sounds could the aftermarket exhaust it came with have something to do with it? or should I check the 02 sensors? it doesn't bog uphill anymore since the fix but the mpg has been consistently 13 miles per gallon and everything I read says it should be more with normal driving.

Although the exhaust configuration can have a lot of effect on the fuel mileage, so can the supercharger. Tire pressures and driving styles also make a big difference. Another issue you may have here is they may have used a lower gear ratio in the differential to give it more power in the SS model that would have a dramatic effect on mileage. Changing the upstream 0/2s could be helpful.