A possibility is that you may put two fans in the same exhaust path, and the one that moves less air will act like a brake on the faster one and slow the airflow of the faster one down. All depending on the length of the ducting that you use. Typically if you place fans in the same stream, they should be matched. If one is further downstream, it can be slightly more powerful.
And using a powerful 250cfm, with an 80 cfm fan, that's really a discrepency. A simpler idea might just be to get one exuast fan, if money isn't an issue, just get a really strong one and put it away from your box, in the attic. If your box is sealed more less, and your path is good, it will pull the air out.
Just some thoughts, your case appears to have about twice the volume as mine does, I have more fans than you, and I feel mine is still insufficient.
Heres a general guide to figure out your needed exchange rate:
http://www.thcfarmer.com/forums/tools.php?do=ventilation