One thing that could cause loss in efficiency and not easily detected, might be intake valve deposits. You might try sucessive treatments with LubroMoly Ventil Sauber. But I am not very confident this is your problem.
It might be worth having someone with a more sophisticated engine diagnostic software program plug in your car and read the various sensor data.
Factory M-Sport package with factory installed M-tech bumpers, factory Bluetooth, factory M-Audio 10" dual voice coil subs, Homelink, M5 rear stabilizer bar, Bilstein B14 PSS coil overs, Stop Tech ss brake lines, Akebono Euro Ceramic brake pads, oem Brembo and Zimmerman rotors, Dinan Stage 1 software, Strong Strut front upper stress bar.