Dan... this subject has been hammered around on this board a bunch... with no real resolution due to no data ever being posted to support the opinions of restricted air flow. It has been said that a bug screen will reduce the airflow by 25 percent.
I have seen no data to indicate the accuracy of that statement. I have used bug screens on every vehicle I have owned... cars and trucks, never have had a problem with overheating, etc. I have run the Lund warm fronts and cold fronts on my Ram... no problems at all, while towing. Last summer, (a year ago), I pulled my hay thru eastern Washington with the warm fronts on, 100+ degrees, with no problems.
I still had plenty of flow to keep the temps normal and plenty of power on the hills. Always remember that a lot of stuff you read here is opinions, based on their experiences. Some people post opinions without experience. This is my opinion.