Doyll knows a ludicrous amount about fans and coolers
, he recommended the phanteks to me. I would also add, again Doyll led, that you remove all the PCIe slot covers at the back, it allows much more airflow under the GPU.
The GPU can definitely leak air to the CPU, the CPU to GPU is almost impossible I'd have thought, I plan on closing off the top of my meshify, and placing a 120mm exhaust fan, just to help ensure the airflow is smooth as possible and running free.
I'm also planning on testing isolating the CPU and GPU. basically a piece of card resting on the back of the GPU and top of the bottom 140mm fan. It might not do much, but equally it may help airflow from each fan, and also prevent GPU heating CPU.