Actually you are going to end up DECREASING your cfm efficiency and here's why:
Adding another fan, especially a mismatched one, is going to add TURBULENCE to your duct system and acutally BLOCK air flow. Clear, straight, smooth ducting is best, with just a SINGLE fan. By the same token, if you have an active exhaust AND an active intake, the fan cfm must be perfectly matched, or you're actually better off just running an exhaust fan to PULL air through the whole system, leaving anice big unobstructed intake hole- preferably in a cool area. If you can bring cool air up from the basement via an unused chimney, or from and iar conditioned room via a duct, that would be nice.