Nice setup, I had a rig a few years back that was nothing but 5x R9 290 all sitting in close proximity to one another. Kind of like your setup. And with the bad cooling in the reference design, I assumed that they would all overheat except the card in front but surprisingly they ran pretty well.

This was mining ETH which if I recall used maybe 250 Watts per GPU, so the heat was obviously much higher than yours, I believe it ran at 80C or so. The fans were running crazy loud but they never had any stability problems or overheats ever. The fans also never died on them.

Had to shut the rig down because as the DAG grew the R9 290 gpus started to slow down after each Epoch, eventually it ran less than 15MH/s using similiar wattage as before and I just shut it down.