And no, you can't kill your heater core either. The whole point of "PTC" heating elements is they adjust resistance rapidly to maintain a certain temperature. You could have no airflow at all through the heater core and current would dial back rapidly to almost nothing once the PTC stones got up to temperature.
The risk of excessive restriction is the fan may not be able to move enough air to keep you comfortable. That is it.
Good Info! I guess the only draw back is you having to constantly adjust the AC vents manually because the car will not heat or cool fast enough inside the cabin due to air flow restriction when both the HEPA and external filter are being used at the same time. I prefer the exterior cover and hepa filter allowing me to use the auto mode when running the hvac system.