First, consider that the purpose of the magnets in a motor are to create a permanent magnetic field. The magnetic flux associated with this magnetic field follows a path from one pole of the magnet, through the air gap to the armature, then through the opposite air gap, then through the other magnet, then around the housing of the motor back to the opposite pole of the first magnet. The magnets are embedded in the motor's magnetic circuit. If you place magnets on the outside of the motor, they will not be in the magnetic circuit of the motor. Rather, one pole of the magnets will be on the housing that the other will be facing out away from the motor. Little to no flux will be added to the motor's magnetic circuit.
If you took the motor apart and replaced the standard magnets with new magnets, then that is a different question. This is a complicated subject and there is no definite answer to your question other than "it depends." Most motors are designed so that the steel in the motor is near magnetic saturation. Increasing the strength of the magnet will drive more of the steel into saturation. How much and what affect it will have depends on the motor and how much you change your magnet by. But you will increase Kt, which means for a given torque, your current will be lower. And your speed will be lower. Your ability to dissipate heat will not change. However, your magnet may be more susceptible to demagnetization. You'd want to make sure your magnet's coercivity (Hc) is high enough.