The minimum duct opening needed for a heat exchanger to function close to its max efficency level is roughly 1/4 to 1/3 the area of its core. More opening will not equal less flow, and at normal driving speeds the flow is already turbulent so additional turbulence will not contribute to less flow either.
I've seen tests on NASCAR radiators where the incoming air speed is 200mph and the exiting air speed on the backside is only 15mph. The heat exchanger's core is the primary restriction in air flow.