I noticed a significant gain when I got rid of the ladder rack from my truck (I had the same brand).
My vote, sell the ladder rack and buy some steel to build a simple ladder rack similar to this:
The main difference being that the front rack is the same height or just slightly lower than the cab. The rear part of the rack would be a little lower yet so on the rare occasion you haul a ladder, there is a slight angle to keep it from hitting the cab. This gives you a rack when you need it without it disrupting airflow above the cab.
Then use the same rack to build a simple sloped cover over the back.