The main thing I would be thinking about is cold soak, and ice on the wing. 6000kgs is the max to land with to avoid ice on the upper surface of an A320. We can take up to 3mm of ice on the lower surface of the wing. Also with an OAT of -15 the fuel you pump in will not be 'warm', so deicing may be an issue. But again with an OAT of -15 your probably going to de-ice before take off anyway, so is this an issue? if you intend to de-ice before takeoff then you could tank up to max landing mass. As long as there are no performance considerations.