Gas mileage isn't bad at all,all depends on how much you keep your foot off the "go pedal" if you know what I mean.I've got 3.73's and 5-speed and I can't complain.
For as longevity is concerned,there's many guys running blower combinations with 100,000 miles plus,so I'll let you be the judge of that
There's no need to re-tune a great running combo unless your are changing something within the combo itself to make it run better yet.