[Johnny Mullet]

Man, that exhaust leak is pretty bad. When my cat started leaking at the main down-pipe, my mileage took a pretty bad hit of like 6-8MPG. My car idles very similar to yours, but maybe a tad smoother. Are you sure the timing is not too far advanced? Does the car "ping" on a hard load like going up a hill in high gear?

Fix that exhaust first so you can hear the damn thing. The timing belt tensioner is noisy like mine is starting to get.