In theory which causes more parasitic drag? A large smooth wing or a small, rough exhaust port?
The wing creates more drag due to the much larger area.

The wing is beautifully streamlined and the flow stays attached to the wing if the angle of attack remains below stall values. So friction drag is the only drag that acts upon the wing, while stubby shapes like exhaust ports have friction drag and pressure drag. This means that area for area, short stubby shapes have way more drag, as illustrated in the figure below from Prouty. But multiply with the length of the wing &amp; stub, and the picture changes.



But the exhaust ports don't have pressure drag, because the exhaust gases are ejected backwards, in the spot where normally the separated airflow would extra drag.