Does this vent cap limit passive air flow?
Plumbing vents work on the principle of water moving and are sized accordingly. Reducing the "net" area may affect your plumbing's venting, thus the drainage. 

We install radon remediation after tests are performed and they indicate it's necessary. Usually it's a passive system with the capability of adding an exhaust fan to "suck" more air through the system if future tests indicates the passive system is not working. Likewise, you could try the covers and re-test for effectiveness. 

We worry about the "amount" of screen on outside grilles blocking air movement for heating systems. The amount of air reduction in your proposed "vent caps" seem excessive...especially if you're worried about keeping birds out of the pipe.