Hot Garage - Do Garage Door Vents Work?
I have found vents like the ones you have in the photo to be basically worthless. I have found that a powered temperature activated vent in the roof is almost as good as an air conditioner. The first one I installed was line voltage driven with a temp switch it was really a huge savings in that home, because it dropped the attic temp by 40-50 degrees, the next couple of homes I used solar powered vents, these were a little more up front unless a new circuit was needed and if a new circuit was needed the solar was cheaper. 
In short a passive ventilation system is limited to the area in square feet you have and the height between them. If I were you I would be looking into a powered vent on the roof, it will work much better in my opinion from actual experience with Louvered vents that were 3’ square compared to a single 8 or 10” powered exhaust that actually cooled the entire house.