The Gibson shorty headers I had installed on my 2001 Dakota 4.7V8 had no impact that I could perceive performance wise, or fuel mileage wise. If there is any change it is so slight to be imperceptible. Probably does improve higher RPM power, but I rarely if ever get over 4,500 rpm so again nothing that I noticed.

Seems little results for anything I changed on my Dak, which I bought used with 153k on it, now at 203k. Five years ago I had to pull the plugs checking out a failed coil pack issue. Found that every spark plug had NO visible ground electrode whatsoever, so the "gap" had increased to well over 0.063", maybe more like 0.125". The spark would have to travel sideways from the center electrode to the body of the spark plug. In a sense, the old plugs had retarded the timing slightly. The truck ran fine, started easily, idled smooth, typical Dakota 4.7 fuel mileage. So I thought new plugs would make it even better, maybe better fuel mileage. Nope, no difference that I could perceive. It just runs fine even after 200,000+ miles.