Did the aux fan run full speed before you added a can?
If not, you've likely overfilled the system. Earlier Bimmers used the AC's high side pressure to set the speed of the condenser fan.....higher pressure ==> higher fan speed. Not sure if E60/E70 does the same, but the pressure sensor is present so it's just a matter of configuring the IHKA/DME to crank up the fan speed as condenser pressure rises.