The main thing to remember is that RPM is not directly proportional to CFM, this means that the speed to airflow ratio is not a linear curve. Decreasing the fan speed a little will decrease airflow a lot more than you might expect. The curve is gradual at first, then drops greatly. Using a baybus is a very good idea because it will allow you to monitor the temps that are affected by the fan(s) you are controlling. If you just use a controller without monitoring, then you run the risk of overheating components.