hmm. thats an awful big room for only 300 CFM. you probably have a volume of about 640 cubic feet right? lets say that 300 is brought down to 200 b/c of the filter, and down another 50 b/c of the ducting. so thats around 150 true cfm that the fan is pulling. im to stoned and lazy to do the math, but i think thats around 1 atmosphere exchange every 4 min 30 seconds or so? whether or not you have an intake fan will come into play here as well...without one you will see a further reduction of true cfm, and an even longer time for air exchange.
what kind of light are you running in there? if you are using more then 400 watts, you will have trouble keeping the temps below 85. i know because i bought a similar sized fan for a grow that was slightly smaller then your 8x10. i had only 1 400 watt light and the room was constantly 95-100 degrees.
for your room, i would suggest getting a fan that can exchange the air once every 60 seconds under true operating pressure. also get an intake fan if you dont have one, and make sure it has about a 100-200 cfm increase in power so that you get good air circulation. i know it sounds like overkill, but i think something in the order of 1500 cfm is needed for the best ventilation. always go overboard on ventilation...
the other, more sensible option, is to air cool your lights. that will reduce the heat enough so that you can use much smaller fans to cool the radiant heat that collects in the room.
