There is one other basic problem with measuring the temperature of the motor on the outside of the can, The hottest part of the motor is the armature... It does all the work... So the can temperature just gives you an idea of how hot the armature is... and it's not very acurate...
If you get ten of the same engines, run them in the same car with the same gearing, and the same ambient temperature, you will get ten very different temperature readings...
Also, with heat sinks, there is no point putting a heatsink on it it will not mak good contact with the can... Also, low airflow areas are pointless for heatsinks.
The best heatsink's I've seen are where they are incorporated into the engine mount... However the must not block ANY air vents on your motor can... This can be really bad, as most motors like to 'breathe'... That is to say, the armature, as it spins, creates a cooling airflow around the can...