I also owned a P4SS and got the same "fume-experience" like you did.
Some thoughts, only assumptions:
a) The fume issue was also the case with your Duratec Plus 4, but her fume-composition was less noticable. The smell of the P4SS engine could be a little more noticable for reasons I don´t know. Could be caused by the metal cat perhaps.
b) I learned when driving the P4SS, that the fume-experience is somehow weather dependent, the fumes are mostly noticable on warmer days with high humidity and almost without any wind.
c) Did your Plus4 duratec had a spare tyre? I guess, that big cut out for the spare tyre sucks most of the fumes back before reaching the cabin.
What I did against it, when driving with side screens mounted it is helpful to open the side screens to avoid too much negative pressure inside the cabin.
