Is it throwing a CEL? I'm figuring you don't know whether you're hitting a rich spot in your fuel curve, right? Are you kicking up black smoke when you hit lag? When you put your exhaust system on did you put all the O2's back? I'm not a Mustang girl so I don't know how many your car has but leaving one off unresistored or having one go bad will seriously affect your fuel...but that's usually across the board and not rpm specific. Also, how do your plugs look?