That is pleanty of cooling air inlet. Looks good to me.
The main aero drag component involved is frontal area. With the AT-6 design, you will have the same frontal area regardless if the cowl is entirely cut out or just cut enough to allow in engine air. (front area includes the entire front profile of the aircraft)
The second component is (over simplified) induced/turbulent drag. That comes from air having to travel over or around or through features. When air has to change direction, it takes energy to do that.
Also, all air in must be capable of exiting. I believe most of you reading this understand that cooling air exit area has to be larger than air inlet area (fill in your own equation here, I prefer 2x).
Only allowing what is requied for cooling air, and leaving remaining flat cowl blocking the air is better than a huge cowl inlet and turbulient air both hitting a flat firewall and trying to flow through a very disturbed path... then trying to exit the cowl through openings less than the open inlet area of the cowl.
How much better???? Well on the AT-6 it might be in 'the fuzz'. But with that AT-6 particular model, all it takes to find out is a consistant engine, a consistant pair of thumbs, a stop watch...... and two cowls
Avoiding turbulent drag is the reason many race cars use air dams in front, and the designers try to keep those as close to the ground as possible. You would imagine that adding the air dam is an increase in frontal cross section area - and it is. However the benifit is worth the trade-off. It is better to have a flat plate forceing the air to flow over or around the vehicle (somewhat predictable and managable flow path), rather than having that air go under the vehicle where all the unpredictable mess/drag/turbulence can occur.
You will see many scale aircraft (corsair, P-47) where actually using the dummy engines will improve engine cooling buy only allowing cooling air in where the engine/cylinder/muffler are located... only one flow path in. Some have noticed this also tends to decrease aircraft drag too.