So if anyone is foolish enough to follow this process....
I decided to experiment hooking too more fans back to front with the existing ones, because they were cheap and silent.
Bad idea. Really bad idea.
On the good side just a rough feel of air flow it worked nicely, the air flow was substantially increased, maybe not double but just by "feel" about that.
On the bad side it is VERY loud. Put close together there is some kind of resonance (or maybe almost resonance) that creates a loud hum. These fans were almost inaudible before, at least at any difference, now the combination could be heard 20-30' away.
So I tried separating them by the width of the wall. That made a lot of difference, the really loud noise only came when they were back to front, but at about 4" distance it was much quieter. But still too loud.
So I tore that apart. I also did a bit of experimenting with a digital thermometer I had previously compared with the zwave thermometer in the closet (about .75 degrees off, varying a bit). The zwave device is at the hottest part of the closet, most of the closet hangs about 2-3 degrees above ambient. The zwave device is purposely at the hottest point, so that is good, but the real point is I think I was fixing the wrong problem, there appears to be adequate airflow to cool the closet overall, but stale air at the top away from the new exhaust.
So incarnation #2: I took those two extra fans and put them under a wire shelf about 2' below that corner, blowing up. My hope is that it circulates the relatively cooler air to that corner, and across the top to the exhaust, better equalizing the temp in the closet. A better alternative would be another exhaust on the other side of the closet, but neither side nor back is really suitable for that.
So now to let it run a couple days and see how well that works.
Getting both quiet and cool airflow in a closet with a PC is not straightforward. :angry2: