Good cooling is important but ideally we'd like to recover as much momentum as possible, exiting that cooling air at as close to free stream velocity as possible. An excellent system will exit the air at above the free stream velocity, offsetting cooling drag or even creating slight thrust.
We'll never get there with a flat air cooled installation, but let's get all we can.
Next step is to instrument your exit with a pitot tube and temp sensor to see what you have for exit velocity.
Might be a case of premature pitottemperaturation.