Most likely yes seeing that's it's a variable fan but just maybe the card requires a healthy digital 1 from the fan (closed loop) to reassure the card that the fan is on, otherwise just check the volts required for full fan speed and ensure that the fan controller can provide that voltage as a max along with the reuired amperage - but I'd still stick to the software to control fan speeds. Worst case, a custom fan speed setting going corrupt in ATT or AtiTool will default back to driver settings.
Besides, if you go for pure manual mode you'll be stuck with wondering what temp you are running currently and when do adjust accordingly, unless you are using an overlay to display temps.