Expensive engines to maintain and not many experienced mechanics to build/rebuild them.

On another note, we don't know what he's done with the car. OP, can you fill us in with the mods you've slapped on to the car? That might help a bit in diagnosing the issue further. So in addition to the catback, did you have an intake or header installed? Pod filter? The ECU should re learn over time and adjust to these modifications, but the stock ECU can only re tune itself to a point. It will still run rich like all stock units to prevent piston failure (fuel keeps them cool). My EG use to blow black soot all over the arse of my car but never had any issues with fuel mileage, or any other mechanical issue for that matter. It did reek of fuel after hitting Vtec a few times. Totally normal. I had IHE on the car with a stupidly large cannon before. All that being less than restrictive will generally let the car breathe better, and consequently it'll also spit out a lot more excess fuel/soot/carbon deposits due to less walls/baffling in the new catback as opposed to stock.