MMD, I really think you're edging dangerously close to the mental health cliff...
On one hand, you say you don't want flash, don't want to draw attention.. But your post screams of someone trying to make some sort of counterculture fashion-statement....
I understand wanting to tone down the flash (I also believe the Turbo is tacky, and needs a careful approach when speccing).. But when you start talking about intentionally accumulating dirt and brake-dust over longer periods of time in some premeditated attempt to obtain a "look", you're being just as vain and conspicuous as the dude in the clean and sparkling bright red car with chrome wheels and the vanity "2fast4u" license plate.
Think of Gothic kids... Trying so damn hard to be anti-establishment, unflashy, unmaterialistic, that they ironically manifest the same exact destiny as those they despise, slaves to fashion, and enormously conspicuous..
If you want a serious non-flash pure-function Turbo that also happens to look nice, then go with something like an Atlas, Meteor or Slate grey, pay to have the calipers powder-coated a less-startling color, do a black interior, and leave the wheels alone (anything you do to the wheels always comes off as an effort to obtain an image, i.e. cars with body-color wheels always look flashier and more attention-getting than ones with simple stock silver or satin). The best way to look serious about driving, rather than image, is to not go nuts customizing the car to look monotone. Don't over-dress, no matter whether you're going for flash, or for plain, either way, don't over-do.
I agree, a car looking dirty and road-worn from a long trip does indeed have a satisfying "patina" to it at the end of the trip. That's REAL, and that's cool, the dirt was part of the journey.
Purposefully accumulating dirt on an extended basis, waiting for longer periods between washes, simply to perpetuate that image... That's contrived, and highly geeky.
Not alot different than a rugby player playing a tough game to victory, bloody, muddy, sweaty, victorious... And then not changing or showering for another week or two because he thinks "I look cool this way...".