Can't really answer for the Stoker, but on the Guru, you have a slide damper to choke the fan if need be. I have the 10 cfm fan and usually set it about 3/4. The higher fan volume allows you to reach the setpoint more quickly, but possibly with an increased over shoot. Since the ProCom Guru has a proportional band adjustment (electronic), I can set it tighter to compensate for the larger air volume to counter the over shoot. You can also control the fan pulse timing to shorten the intervals at which the fan runs. Having the larger fan is also nice when you want to leave the probes off and just use the fan to quickly ramp up for a high temp cook.
Naturally I would only get the fan in a size they recommend with their unit, but if they offer a 10cfm, I doubt you would regret it. I am currently using my 10 cfm fan on a smoker much smaller than the KK and the results are just fine. Just my opinion, but I think it gives you more flexibility provided the Stoker combines the right controls to use it. But of course the 4 cfm or 5 cfm would also be more than adequate too.
-=Jasen=-