Are there any disadvantages - apart from initial purchase cost - to over speccing a BLDC motor?
BLDC with higher torque rating will differ in the size, thus it will have larger inertia, making the motor less prone to the fast dynamic change of speed, position.
If your driver can fed the motor with less than rated current the overall torque will be smaller than rated. The only constraint is the match between driver and motor - the driver has to have the capabilty to drive the motor within the range of winding inductance.