i have a very similar story here, 33 with 11 psi boost, 3.5 turbo back, fmic,
i cleaned my afm with contact cleaner and that cleaned it up to near new, it was filthy, (had a crappy hks pod filter which filtered nothing, recently changed that to a 3a racing.) i also siliconed up my coil packs, which seemed to help with the flat spot, so check your coils.
i think my coil packs are on the way out maybe there not giving the spark plugs enough voltage for a decent spark, hence a crappy burn. if i change gear up in the rev range 4000 rpm + i will hear a pop from the exhaust. i seem to be getting worse fuel economy as well, i'm looking at 300km a tank now, i had a look at my O2 sensor and its got bosch written on it so i think its been replaced. so i'm not going to buy a new one.
the only thing left is to get consult cable + software to see what the computer is doing,