I'm no scientific genius myself, but is it possible that in non-DI designs that there is enough unburned fuel remaining in the exhaust to carry the soot out without deposition? I mean, I am getting 30% more mpg out of the same size engine - compared with my 89 Accord's port injected 2 liter. So, there CANNOT be more unburned fuel coming out of the exhaust on this car. Maybe soot is all that is left from the combustion process now and there isn't much else along with it to prevent sticking to exhaust outlet. Another possibility is that the unburned fuel retains carbon within hydrocarbon compounds rather than having it ejected as elemental carbon.
Following this line of thinking, the black deposits on the tailpipe may be a good thing. Soot is heavier than air and tends to settle at ground level while other exhaust contaminants contribute to higher level smog. I distinctly recall my 72 Pontiac having exhaust that smelled like fuel - a little. The exhaust from the Mazda3 (Skyactiv) smells like nothing. It may even be possible that the ethanol in the fuel contributes to the remaining soot. I'll leave that to the REAL scientists and chemists here.
