[Christ]

The longitudinal bars aren't going to make enough difference to get outside static noise, but it will help unless they serve some other innocuous function to aerodynamics [there's a car that removing the outside mirrors actually makes the aero worse, etc].

On some things, you have to choose between utility and economy to some extent. If you never use the roof rack bars, remove them. If you honestly will use them at some point, leave them there. They ain't gonna hurt anything.

Belly pan is a good mod that is, as you say, basically invisible. It will also help with long-term resale value in some cases, if that's a concern to you, since not only does it protect the engine bay from underneath, but also the entire underside of the car, from dirt, rust, and corrosives.

Wheel spats are also 'basically invisible' because they install under the car behind the tires, and a modest lowering and/or a front 'lip' on the bumper also provide reasonable gains for little expense and will be 'basically invisible'. Between a modest lowering, a belly pan, wheels spats and maybe going up one size on tires [due to reduced aero load] you may see up to a 15% overall gain in fuel economy [depending on the driver and overall configuration, of course] without any/much expense and very little 'noticeablility' increase.

The reason for going up a tire size is because you're reducing aero load and your car was designed with a certain tire size both for aesthetics and to account for average load while cruising. You've altered that load, so your engine will be operating in a less efficient range of the BSFC map while cruising. Going up a tire size will increase load without increasing rolling resistance [generally speaking], and also fills the wheel well slightly better, for added aerodynamic benefit.