Those are expensive mods for rather little gains in mpg.
They help, but they'll never pay for themselves in fuel saved.
I'm spotting the
racer-approach to reducing fuel consumption.
No offence meant by that - we see it more often when folks join up.
The mindset that you need hi-tech mods to improve certain aspects of a car.
Well, as it turns out, you don't always need that.
There's little difference with higher octane or premium gas unless your engine is set up for it. And even then, it's marginal.
Once again, the fuel saved by "premium" gas - if any - won't ever offset the price difference.
Another mod that's expensive for little gains.
Do it when you want to drop the car anyway, but not specifically to get spectacular better mileage.
I'd look into changes to your driving style, rather than technical modifications to the Mustang.
Have a look what you could change :
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
Try to get the same average speed with a lower top speed.
The endurance racing mindset.
Smoother yet faster.
Stealthy mods like a grill block (can be done on the inside), air dam or even belly pan (bit more involved to build) are always options if you want to keep the looks of the car unspoilt.
Another option to consider is reserving the Mustang for the fun rides, and get a cheap-ish but far more efficient car for the boring rides.
If you do a lot of miles, the beater can end up paying for itself.
(It's a rather specific situation, but my
new car will pay for itself in like 10 years in reduced fuel costs.)