sounds more and more like a clogged cat. you can pull the up stream o2 sensor, if it runs better ( with this exhaust leak) the cat is clogged up.

you can also drive it at night, get the car good and warmed up, get on it some, then pull over and look to see if the cat is glowing red, if so its shot.

you can also when the car is cold, bang on the cat, if you hear it rattling inside, its also shot.