strange u get better fuel econemy running it harder, than putting around, my car is total opposite. a little black smoke is normal from a highly boosted engine, as where then it would be practical to run it a little richer.
seems hes running it richer, so it doesnt lean out up top. leaning out is bad , but of course running rich alot is bad also, over time, it will wear out ur rings.
maybe try a new tuner. should be able to rectify that with ur PFC i would imagine.. just gotta find someone who knows how to tune the pfc properly.