Todd Marcucci has an excellent write up on the MAF posted on his website

Scroll about a third of the way down to read about checking the MAF.

Hmm, weird. Like you said, light soot coating (possibly rich?) yet mpg is way up. I wonder it your ECU is enriching or leaning out, out of spec at various points in the rpm range? About the only way to find that out (that I know) is to strap it on a dynometer, to get a air/fuel map ratio chart.
Interestingly enough, Todd and I discussed the pros/cons of installing an aftermarket fuel/air computer (like the Apexi S-AFC) on a 2.6l. Afterall, the sport impact crowd are religious about retuning their ecu's whenever they perform any mods; to maximize the gains. Only makes sense that we could and should do it to. But while the Apexi unit is reasonably priced, dyno time is not!

Starting to get stumped here, might need to call in the big-guns on this one.
Mike, Dan, Todd, or Gary Lemond.
Let me know what you think.

Travis