What gears are in the rear axle?
If you look on the drivers door sticker it will have an AXLE code number, my '94 w/4.0l is an R6 axle, so L/S(limited/slip) 8.8", 3.73 gearing
Gearing doesn't give you more power but it changes when you feel the power, lower RPM or higher RPM.
Spin the tires all day long but with top speed of 45mph, lol
You can see the code numbers here:
http://www.therangerstation.com/tech_library/Axles.html
Vacuum leak would show itself as a high idle when warm and often a pinging noise from a too lean mixture, does it do that?
Power and MPG are on opposite side of the same coin, the fuel coin.
A gallon of gas, regular or premium, has a fixed amount of energy, nothing can change that.
To get more power from an engine is easy, burn more gas, that's pretty much what all power add-ons are about, even your cold air intake.
The cooler the air the denser it is so more fuel can be added, MPG goes down but that is the point of adding a cold air intake.
They don't really work that well, at least not better than stock intake so MPG won't suffer that much, but they can "sound" better.
Turbo-chargers, super-chargers, higher compression, bigger cams, all have that one thing in common, burning more fuel per stroke, power goes up, MPG goes down, pesky laws of physics, lol.