You have pretty much described the meaningful bolt-ons, I generally recommend the following, in this order:
- Open up the exhaust, 2-1/2" from the manifolds to the back bumper. Install an H- or X-pipe per your pleasure, the only practical difference on the street is the sound. H-pipe = Musclecar, X-pipe = NASCAR. Select "off-road" or catted, again it makes no practical difference--o/r pipes are louder.
There are many that say chambered mufflers like the Flowmasters sound better with H-pipes, and that straight through mufflers like Magnaflows sound better with X-pipes. Having owned both combinations (and scrapping the H-pipe and Flows) I am among them.
- The next thing to do is to buy a tuner, handheld with "tunes for life" or a more sophisticated system if you plan on doing your own tuning. The stock tune was designed to make the EPA happy, run on 87 octane swill, and insure that there was no way a GT owner could possibly hurt the engine, regardless of how hard they romped on it. The ignition timing is absurdly conservative, and in open loop it runs pig-rich; especially the '03s and '04s with the pink 21 lb/h injectors, that will dip into the low 11s and even high 10s AFRs at WOT.
- Then if you want to wake up street performance change the rear-end gears. The ratios 3.73:1 and 4.10:1 are the most popular--I will not get into which is "best" as that is a very subjective discussion. I will say that I got 3.73s when my engine was near stock, and if I had it to do over again I would have installed 3.55s or even stayed with the 3.27:1 stock gears.
- After the above install an aftermarket upper plenum and 70 mm throttle body. Again it really doesn't matter which maker--read more about that here. Some will say get a 75 mm TB if you plan on going f/i some day, to which I say nonsense. In the first f/i and n/a are distinctly different environments with regard to intake flow, and in the second a supercharger or turbocharger will cost so much that another $100 will be a ****-hole in the snow by comparison--and you can easily sell that 70mm TB on eBay for $50 to $60.
Beyond the above there really isn't much to do in the "bolt-on" realm. CAIs (Cold Air Intakes) are a complete waste, some actually hurt engine output. The stock intake is a CAI and a very good one at that.
Things like underdrive pulleys, lightweight flywheels and aluminum drive shafts can improve acceleration, but only if you are a "drive it like you just stole it" (DILYJSI) )sort of person. I have those things, and must admit that upon occasion, under safe conditions, I am a DILYJSI sort of driver.
Don't waste money on after market COPS, fancy spark plugs, cotton gauze air filters, special oil treatments or anything where the person selling it presents claims that don't make sense. The reason they don't make sense is because they don't make sense.
If there were some magic elixir or device that could actually improve fuel economy or power the automakers would be buying or licensing it to make the Corporate Average Fuel Economy higher, or to give them bragging right over the competition. They don't because there is no magic or snake oil.
Above all keep in mind that of Cheap
you only get to pick two.
BTW, I lived in Plymouth for 45 years--been down here for 20. I miss crisp clean air; there is a permanent musty smell here that you do get used to, and having four seasons; we have 2-1/2, hot friggin' hot, and nice--but I do not miss scraping and shoveling--not one bit...