It is true that the 3.8L does not respond great to mods, but it does respond.
For a CAI, exhaust, Under drive pulley, and a tune expect about 10hp MAX from each if you are lucky. Say you did all of the above, i would be happy with 30 more hp.
With an auto transmission a tune can really wake it up changing how it shifts and fluid pressure etc and that makes a big difference.
All in all I say you can shave a full second off your cars stock quarter miles time (15.6 manual 16.1 auto).
Rear end gears add NO power but make a GREAT change in the car, 4.10 for auto 3.73 for manual. And do a Tlok unit (so both wheels spin) when you do gears, gas mileage is hurt doing gears though... by about 2mpg highway, but you gain 1 mpg city.
Stock you are at 135rwhp you should be able to get around 160 without to much invested. Manuals have about 15 more rwhp to 20 more stock.
So thats what you are looking at for basic bolt on modifications, if I didnt list it then... dont do it
If you want more you can build your 3.8L into a 4.2L stroker engine (F150 parts) and port the heads/intake and have a custom cam. The end result will be about 250 rwhp/tq which isnt to bad.