Genghis raises an important point about the static ports, and it also applies to the pitot head(s) as well. In the original configuration,was there any flight testing carried out to determine PEC`s, was it checked with the manufacturer, ie approved?If not then you may have to seek their advice. There also may be a CofG effect depending on the position of the equipment as well. To exaggerate somewhat,you will have minimum trim drag with the a/c at an aft CofG,and more at a fwd one.This may mean in doing s/e climbs especially in "light" twins that you may well be using more control and hence have more drag.
There are references on this forum as to carrying out PEC`s in a/c not equipped with full test insrumentation..Genghis will offer advice I`m sure, as he`s doing a thesis. BS