I don't think it's really an issue of drag as much as it is lift, or reduction of lift. Remember the 370z is designed to have zero rear lift. This is probably done with the spoiler and flat (err... flat enough ) bottom. The fact is that none of us likely have the resources Nissan has in order to determine what effect there is from the various styles of aftermarket exhausts. All I know is from most of the articles and papers I've seen regarding race car aero, airflow underneath the car is at least as important as airflow above the car, if not more important. Just look at some of the supercars coming out now that generate hundreds of pounds of downforce without using a wing. Of course they usually have a completely flat bottom along with a bunch of other specific design considerations, but you get the idea.
That said, I think anybody really concerned with the issue would probably also think to buy or build an exhaust that's tucked up as high as possible in order to leave room to build a custom diffuser. Since we don't necessarily have the wind tunnels or computer modeling at our disposal, we really would have to stick to the academic. My gut says that messing with the exhaust in any way risks creating some rear-end lift. If that's a concern then you should plan on adding a rear diffuser.
But, I really think the only folks that ought to be concerned with it are those that heavily track their cars and want to do so with no rear wing or one as small as possible.
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