Were not talking a Header , were talking a TB thats 8mm , .310 thats about 45-48 sheets of note book paper larger then stockIts like slapping a header on the MK3 and not tuning for it, going lean is a bad thing. You adjust the tune for the increase in TB diameter for higher airflow.. common sense when tuning for a larger TB...
My goodness just how lean you think it would of gone , ECU can adjust for fueling for small % , No Tune was needed to test the TB
Cant believe your comparing a header to a 8mm larger TB as for whats needed for tuning
Like I said you sell these so sure there is a gain