simple exchange is not possible as SP20 uses a very special and tiny PWM Connector.
This connector seems to be a 1.5mm JST ZH 1.5mm 4-pin
(http://www.ebay.com/itm/Micro-Mini-JST-Connector-1-5mm-4-pin-w-Wire-x-10-sets-/161350482183) not 100% sure but 99%
to use the new smarter fan control you either try to order theses connectors or disassemble the stock fans one and solder etc.
as Sp20 comes with a high power 120x38mm fan that is specified up to 200 cfm and this amout of air may be needed in the next summer, I think we need to differ the CFM classes and target GH/s.
-1 safe as stock up to 1700 GH
-2 safe for medium hash rates and conditions
-3 safe for green underclocking (below 1100 GH)
class3 will be an easy one, many low flow 120x25mm fans will suffice and be nicely quiet and can be driven from external PSU.
tested replacement fans class 1-2:
Delta ? 1212 (afaik well known server fan?) high CFM
Scythe Kaze Ultra (38mm depth, medium-high flow) PWM?
are both really quieter at same flow?
Did anyone tried to stack fans in the fan-space of the sp20? there is a lot of room for bigger ones?!
please feel free to add your experiences like the firmware wish list!!!
I don't think 1700GH is practical unless you have free electricity, you could just buy coins instead of exchange electricity, heat and noise for it
Delta fans are very good for server usage, previous king of high CFM fan
http://www.xtremesystems.org/forums/showthread.php?261778-120mm-Fan-Testing-on-an-MCR120-Radiator-Round-6