from what i understand a MAF cannot see or detect boost (air under pressure) and therefore all that extra un-metered air leads to a lean mixture. this is why a MAP is more suitable for turbo applications. relocating the MAF also creates problems (learnt from experience) if the BOV isnt recirculated as the ecu meters fuel for this "lost air" causing bogging down due to extreme richness on gear changes. also, when the ecu sees this amount of air it advances engine timing which is the worst thing for a boosted engine, causing severe pinging! the idle in my case was also horrible. by relocating the MAF the ECU 'sees' exactly how much air is entering the engine and meters fuel to suit. i had to re-relocate the MAF back to the throttlebody when i decided to go fifth injector and pressure switch. the big plus here is that off-boost the car drives normally with great mpg and torque curve. the switch into boost is quite dramatic! hope that answers ur ques.