Not disagreeing with what has been said here as it is illegal and more importantly unsafe to do as your LHS owner has done... but have you checked any of your other planes for similar performance problems? I would not recomend flying them but if you still have the old transmitter do an engine running range check on one of your previously "OK" planes and compare the two transmitters. Please report back to us if there is an apreciable range loss as this will further the validity of properly getting a transmitter retuned by a licenced technician.