1995 Nissan Pathfinder gas mileage

Engine Performance problem
1995 Nissan Pathfinder 6 cyl Four Wheel Drive Automatic 135k miles

last month I changed my exhaust on my pathfinder and I have noticed an extreme drop in gas mileage and the way it runs. But I didnt notice it until about a week maybe a week and a half later. A technition said it may be the coolant temp sensor not telling the ecu when it has warmed up there for its not cutting back on fuel concumption. All I want to know is if I put a better flowing exhaust system headers, high flow cat, glass pack, into fake duals if it will cut back on my gasmileage by about 6 miles to a gallon.