[Ecky]

From GFchips.com:

The ideal air fuel ratio (mixture) is termed stoichiometric. This means that there are 14.7 parts air to 1 part gasoline. By achieving this air fuel ratio an engine is able to run more efficiently, meaning better gas mileage, better emissions and more horsepower. Through rigorous research we were able develop a system that adjusts the factory ECU's settings in order to produce this ideal fuel mixture.

When you are full throttle your engine load is increased dramatically over normal freeway cruising, so the chip tells the ECU to compensate for the larger volume of incoming air by injecting more fuel into your engine and advancing the timing, giving you increased overall horsepower
When you are cruising on the freeway or in the city the load on the engine and incoming air is little so the chip tells the ECU to use less fuel in order to achieve maximum fuel economy

The factory ECU already does this in all cars, and the VX does it better than most cars in part because it uses a very sensitive O2 sensor - hence the $300 replacement costs. The VX's lean burn intentionally leans out to 18-22:1 under low load conditions (lean burn) to save you even more fuel.

Honda has tuned the system such that it doesn't burn any valves when running these super-lean AFRs.

Ideal ignition (spark) timing can only be utilized once ideal fuel ratio has been achieved. By advancing the spark timing the piston is allowed to have a longer power stroke, and achieve a more complete burn of the gasoline vapors inside the combustion chamber. These factors are essential in creating more efficient power for a smaller amount of gasoline.

Due to government regulations on emissions and power, many manufacturers program your car's ECU with very modest fuel and spark settings that negatively influence your car's power and gas mileage.

Advancing ignition timing can also be achieved in your car by loosening the bolts on the distributor and advancing (turning it toward the firewall) ~2 degrees. This should probably be done with a timing light (and not done lightly), as advancing too far can have some pretty serious consequences (such as a blown motor), hence Honda shipping them from the factory with somewhat conservative settings.

That said, your car is an extremely fine-tuned machine from the factory and almost any aftermarket modification to it will just make it worse.

EDIT: If you want a mod that will improve gas mileage and improve your car cosmetically at the same time, I'd suggest lowering it ~2".