yes, you could spin the blades faster by reducing the size pully on the fan or increasing the size pulley on the motor. depending on the cost of the pulleys vs. cost of the belt, you might be able to reduce pully size on fan, increase size of pulley on motor and not need to change the belt if you have enough adjustment on the belt.
just for reference and I really have no idea what size pulleys you have, but let's just say the fan has a 5inch pulley. if you replaced it with a 4inch pulley it would spin 25% faster. before the internet engineers bash me and tell me you wouldn't really get that because of increased drag/deadhead etc., they are right, but i'm guessing you just want more air and are not going to get out an anemometer to measure to make sure we are right?
BTW, I have the 2speed 42" commercial electric from HD and it moves plenty of air but it is so loud, you almost need ear plugs to work around it and need to shout to the person 2 ft from you.
