No... you are repeating bad info based on confusion between volumetric efficiency and fuel efficiency. Volumetric efficiency is a measurement of the engine's ability to get as much air into the combustion chamber as is physically possible. At your highest volumetric efficiency, you are also at max torque and this happens to be when your engine consumes the MOST fuel.
Very quickly I will say that on poorly designed cars like US domestic vehicles, changing the intake and exhaust can slightly improve fuel economy. But on our Hondas it does not.
If you get yourself a real time fuel efficiency gauge, you learn very quickly that your car does not get that average 32 mpg all the time that you're driving. When you're taking off from a stop, gas mileage drops to around 10 mpg. When you're cruising, your mpg might be 45 or 50. When you left off the gas in gear to coast down, your gas mileage will be infinite for as long as your RPM's are above around 1500. Knowing this you can begin to understand how certain things might improve your mileage.
Taking off from a stop there is little you can do to increase your potential fuel economy. The displacement of the engine is set from the factory, and Honda has already given us the lowest displacement you can have (while maximizing power output) and still have a good driving experience. If you live in a hilly area, then a turbo or some basic N/A mods will increase your mileage. But most people will never see any gain. The reason your mileage increases is simple; you take less time getting up to cruising speed... less time at heavy throttle where the fuel map goes overly rich to protect the engine from the heavy load.
To maintain a cruising speed requires as little as 10 horsepower! The engine simply has to overcome the wind drag, friction of the tires, and the accessories running off the crank pulley. Wind drag becomes a big issue above 50 mph. No performance mod will improve mileage here, because if you get more air into the engine, you simply back off the throttle. Keep in mind that the throttle limits air flow into the engine. Don't think that engine mods improve air flow at all times... it only improves air flow at WOT and at high RPM. At medium to low RPM, the engine is only capable of drawing in a limited amount of air and the stock intake pipe is sufficient. (It's possible that the turbo restriction in the exhaust actually improves mileage but I have not personally tested this).
Under deceleration fuel cut off, there's nothing yo can do to improve mileage beyond infinite. You simply want to maintain this state as long as possible by downshifting when appropriate to keep the RPM's above 1500 keeping in mind that the lower gears increase engine braking. No performance mod affects this state.
To improve your fuel economy you don't waste your money on engine mods. They will NEVER pay themselves off. Adjust your driving technique and keep up with maintenance. If you can cheaply improve aerodynamics by removing the spoiler and mudflaps, covering under-body gaps like the one opposite the muffler, and installing a front lip, do so. If you have a programmable ECU for performance reasons, target the area of the fuel and ignition maps where you are at cruising load and RPM and lean the mixture while advancing the timing. There is little chance of blowing your engine under light load, and I've found that Honda engines can go to 16:1 AFR pretty easily with the right timing (lean mixtures burn slower and need the spark to ignite the mixture sooner).
