In my “been a hot rodder, mechanic, and horsepower junkie for 40 years” opinion, searching for better fuel mileage is never, NEVER cost effective. The items that you mention and others, tuner, CAI, headers, free-flowing mufflers, catalytic converter removal, air dams on the front of the rig, etc, etc. just aren’t cost effective. You will have to drive literally hundreds of thousands of miles to make your money back.
Horsepower, however, is a different way of looking at things and often shows a fuel economy return as well.
Probably the best bang for your buck is a tuner with a custom tune created for your rig. This will involve logging some data with the tuner and having the tuner company create one that is beneficial to you. If you plan on intake or exhaust modifications, do them first. Otherwise, a new tune will be required due to differences in the way that the engine breathes.
Tuners are great tools for getting your engine to run at its peak efficiency, which is sometimes not what the factory intended for future sales or emissions requirements. Because they can alter the output of CO, they are consider “pollution control altering devices” and illegal for highway use. This can be an issue in some areas.
