I've thought about this...
I have a 240 AIO and an air cooled GPU. While having the GPU and CPU under full load at the same time, my CPU temps can increase by about 5c, and liquid temps go up about 2-4c. I might try the other orientation you're talking about, but I think the amount of air that can come through the bottom wouldn't be great, and might cancel out any benefit you'd get
Edit: Also, would it be better to have two slim fans in between the radiator and the bottom panel pushing air into the radiator, or have the fans in between the radiator and the motherboard, and use one slim fan and one full sized fan? So many questions, not enough time.