GTI 337/20th Anniversary :: Exhaust Leak And Bad Gas Mileage

Some questions I have regarding he bad gas mileage my car is getting. Up until winter, my car (2005 mk4 gli) was getting 300-330 miles to the tank. I thought was reasonable, about 150 to a half tank. The setup is pretty normal, apr stage 1 tune (87,89,93,100 octane) with bolt ons (upgraded n75, DV, things like that), a cat back exhaust (unsure of brand), 90k miles was just passed last week. Timing will be done in the spring.

The car has no stutters upon acceleration and idle is staying pretty consistent about 900 rpms. I know there is an exhaust leak, at the flex pipe as I seafoamed the car this fall and discovered that. My question is, which down pipe to you suggest to be the best for the money? I've seen a bunch of different brands, weather it be 42dd, ecs, places of the such.

Would it be cheaper and more efficient to just get a turbo back or stick with the cat back and stay happy? Also, I'm assuming if the leak is in the flex pipe, that the down pipe needs replaced all together. The car DID have a check engine light, for the leak. Since winter the cel has gone away and the gas mileage dropped drastically. When I get a new down pipe or fix the exhaust leak, I also want to change the rear O2 sensor. I believe personally this maybe causing my gas mileage info.

Lastly I have a fuel filter to replace the oem one that's been in there. The same thing happened to me last winter and when summer came around, the gas mileage went up to normal 300-330. I drove to h2oi and was getting well over 30 mpg. Now I'm lucky to get 18mpg.