well I was warned by a person who worked at Motec that there can be a pressure differential between the two sides of multiple throttles. If your running an airflow meter this would tottaly erradicate this problem. As for the RB guys I will take an educted guess here and say that it woudl firstly be much easier to bolt up a new manifold with a 100mm thottle body than to try and bore out the factory throttles and fit new butterflies. Secondly I think they do it to make tuning easier but hey I could be way off on both these points.