I was thinking about this recently also. Is your 15" fan just circulating air, rather than pulling air through the inlet? In a small tent the fan is definately going to accelerate air moving over the plants thus removing heat faster than normal, same reason we feel cooler under a fan but the ambient doesnt actually change much. Exchanging the air of course will bring your tent temp closer to the external ambient be it warmer or cooler.
In an attempt to keep the night temp from dropping too low in my cabinet (inlet is ducted to bring in fresh air from outside) I had to stop the fans running overnight, instead im using some timed, periodic, bursts to flush the air every so often.
I'm running on led so don't have issues with heat, but if too much heat is the issue then moving enough cooler air in the bottom and hot air out from the top will give the most uniform cooling, blowing at the plants alone will remove heat from the plants, but how much depends on the air speed and orientation, which will vary across the area.
The HPS is doing two kinds of heating its radiating your plants and also the assembly is heating the air around the lamp which will naturally go to the top, Best do a good job of removing it there, even better than using a fan just to circulate the hot air across the whole tent. I'd say let it congregate at the top away from the plants and get it out asap.
Turned into bit of a long post sorry
