it sounds like you have very good airflow, but does the 2 inch pvc elboes suck the heatted air off the bulb of the hps? i have found that if the elboes are positioned just the right way that heat from the bulb is sucked away by the ventilation system, i had a 400 watt mh in my cabinet, with no glass cover, and doing it this way kept my temps down to low 80`s.
you could also add a small intake fan and duct it to the hood from the flowering side to bring in the cooler air from the room, but you must install a flapper type fan so it only lets air and light in when light is on in the flowering chamber.
i hope this helps
any questions about how to and such, just drop me a line and i`ll try and help


P.S.
i changed my flowering chamber light to 500watt compact floro, it cut my heat build up down by 15 degrees, now the temp never gets higher than 75 degrees, and it gives off more than enough light to grow a few scrog clones