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.
540 M-Sport package (Canadian market), factory Bluetooth phone kit, factory M-Audio 10" dual voice coil subs, Homelink, M5 rear stabilizer bar, H&R 15mm spacers, 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, BMP strut tower reinforcement kit, USA Spec Ipod/Iphone integration.