I've noticed other 5th gens with dirt collection/staining on the rear bumper above the stock exhaust outlets. It's happening to me, does it happen to anyone else?

If it's black you are running a tad too rich.

what does that mean?

Too much of a lead foot...

means to much fuel going into engine not enough air.. get an intake or CLEAN YOUR AIR FILTER!!!

The air filter only has about 1k miles on it, it's spotless. No plans for an intake for this car, probably just a K&N panel filter from my previous Max, unless this soot keeps up.

I've been getting well over 300 miles per tank, even with this crappy "winter gas," so I doubt it's my foot, not too many WOTs.