On my case, the side panel fan is a Kaze Maru 2, a 140mm fan set to intake. The side window is 120x120mm, with 120mm fan mounting holes. But the KM2 has 120mm mounting holes, and its 140mm fan takes up the whole 120x120mm square window, where a 120mm would take up less than 78% of the window. Better use of the space. In the diagram I left the choice of intake or exhaust open because some gpu's blow hot air in all directions. With lower front and bottom intakes the system could afford a side exhaust with such gpu(s).
In my own case I used a nibbler to eliminate the pillars from between the slots in the backplane. Now I have an open window there just as I have an open window up where the rear grill used to be.
I believe that removing the rear grill is key to developing what I call a positive flow case. With such an open area it cannot really build up a high pressure. Is it effective? Once I did an experiment. I removed both top fans and left those grill-free spaces open. In the other spaces I either removed fans or left them turned off. The only fans running were a pair of YL D12SH-12's acting as push/pull on a Megahalems. Even with the top open it was able to hold up a Kleenex over the lower front fanspace. And here is a picture of the power of a heatsink and an open rear grill space:
BTW -- this is a $40 case.
I'm toying with putting a plastic separator between the lower section -- the gpu space -- and the upper section -- the cpu space. Hobby shops sell sheets of stiff styrene for model railroaders to build houses. The stuff works great for all sorts of things.
The reason I am not concerned about older experiments where you say "sure, you cpu will get another 2c of cooling, but . . . " is that in the Intel world, since the LGA1366 there is nothing left on the motherboard that needs cooling. Lynnfield took the last of the hot chipset components and put it on-die. The mb components not only don't need the cooling anymore, they are in a position tp compromise the cpu cooling. And with an i7 8xx cpu, you really need to cool that. (And yes, I am aware that with each passing tick-tock, the need for even heavy-duty cpu cooling is passing.)
That leaves the gpu, which is still very hot on gamers' machines and is(are) in a position to really mess up the cpu temps with not very much gain for gpu temps. Hence the thoughts about partitioning the plenum into gpu space and cpu space. It also explains why I think slot pillars have to go.