on the shocks, its not because of lowering the vehicle, you are right that it would decrease the value of the airdam and undertray, the reasoning is that as shocks get older there effectiveness decrease allowing excess movement which can rob your vehicle of power (shift in weight distribution when accelerating, turning corners etc) long story short, a stable vehicle gets better mpg. as for increasing air volume in and out, it doesnt necesarily increase fuel consumption, if you were to take an air/fuel meter and check your ratio you might find that you have a rich mixture, this is very common on older american engines and can be improved by decreasing the restriction on the airfloe into the engine (also a smoother flow increases atomization of the fuel allowing a more efficient burn process) as with the exhaust it just decreases the work the engine has to do to "exhale" but the biggest thing that is an engine specific affect is the maintenance.
anyhow, try the other mods first, those are for the most part the bigger helpers, the intake / exhaust stuff is all afterthoughts
happy modding
john