short answer, yes
Really? Why is that or is it been too long a day to explain?
your bonnet is an enclosed area, almost like a sealed box, there is no where for the heat to go. so at speeds below say 40 mph there just isn't enough going in to do anything, at high speeds of about 70mph, then yes it will have an effect, but you'll still have heat soak.
think of the heat coming from a tumble drier vent sealed in a large box, and then you blowing through a small hole. if the heat was leaving/cooling then yes the vent helps, but the issue is the engine is continuously heating the area. and in hot weather like this it make sit even worse.
worst case scenario is you'll see the engine bog down in slow traffic. basically the engine sucks in a load of hot air and the engine stutters for a second.
Thanks RTBISCUIT that's a very good way of explaining it. Should speak to you more about my problem solving!