hu13cap, while your explination of convection is fine you have missed some points. Looking at your design, yes you have fans extracting but none supplying. If you are not putting air into the case under force, but extracting with force (your fans) you will create a negative pressure inside your case and your fans will be less effective.

Going into the physcis, if you have a negative pressure it means you have a smaller voume of air inside your case, meaning less medium through which to absorb the heat and trake it away from the components. (Like running a water cooling setup with small diameter tubing vs large)