ugh...no data logs but I can show you many soot covered rear valances that are cobb tuned . Like I said I'm just not a fan.
Soot covered rear valances are an indicator of the stale or turbulent air at the rear, not necessarily indicative of AFR. Of course, when a vehicle is running an aftermarket tune, I would also expect it to be beat on (WOT often) by the operator which does indeed result in more exhaust soot collecting on the rear of the vehicle, no matter the tune. Also see previous comments on turbocharged or other forced induction vehicles. Opinions of AFR based on such visual clues are at the least false and at the most, misleading to others who are informed of the false opinion. Nothing personal, just stating the facts as an experienced "tuner" using Cobb and other brand hardware and software.