Tunes and downpipes are the biggest bangs for the buck. The best way to "keep it quiet".
Fuel - the only tuner I know of who allows for different octane levels is Matt of Gearhead. You should talk to him directly.
Intakes - generally power is generated at 4,000 rpm or higher, so everyday driving - a bust in that regard. But plenty of intakes that are great eye candy, the K&N of course is ear candy as well. The cheapest performance gain in the quarter is a K&N dropin filter (0.1 second), Airraid with the weatherstripping truly sealed to the hood (0.2 second). Testing done on SHOs only.
The 160 TStat is not 100% necessary, as brucelinc has proven. But many will swear to its' efficacy.
Catch cans - can be done cheaply as DIY projects, best mounting location for the can is in the coldest location. But apparently if temps drop below freezing, that can be a problem as well. So choose location wisely.