I laughed when I first started reading this thread back in Aug. shortly after I bought my 997S....but this morning I looked and just like yours there is more soot on driver's side. I replaced my stock exhaust for PSE at 500m and went and looked at originals and they were the same. I have 900m on PSE....Given there are at least three of us with more deposits on the driver's side I would assume its a nuance and nothing more....I would be a bit more concerned if we didn't have exactly the same differences. ...my pipes look exactly like yours....I have only used 7oz of oil in 1450 m.
I doubt we will ever get a definitive answer....most probably wouldn't notice this and by 6000m there would be enough soot on each to LOOK the same....perhaps driver's side bank of cylinders run richer at start-up????
I share your OCD pain!!!!...and I'm a psychiatrist!!!!

Gray