Cooling fans and CFM

I am a little confused about the CFM rates of some of the cooling fans suggested for going turbo. In the Turbo Guide it is suggested that a TOTAL of 2000CFM is desirable, fair enough, Most of you guys on here seem to be running 1 or 2 12" fans. I have looked around and here in Australia the most readily available quality fans are from Davies Craig, however their 12"- http://www.daviescraig.com.au/main/fans.asp?action=model (model DCSL12)as far as I can see by the graph only flows up to about 800CFM as does the SPAL- http://www.spal-usa.com/fans/automated/tech_sheets/0375-0384.pdf I have seen other fans posted on here in various different threads flowing up to 1600-odd CFM how can there be such a huge difference? Will a single Davies Craig 12" be enough to replace my standard fan, whilst retaining the standard airconditioning fan? what if I went with their 14" as suggested on their ‘car make’ option? the 14" would only mount in the engine bay though as there would not be enough room in front of the radiator. or should I get 2x 12" for in front of the radiator and leave the air con fan in place?