You're right, the DIPs are not designed for heatsinks like this, but one of the jobs of the casing of any chip that generates a lot of heat is to dissipate that heat.
Right. But most MOS chips don't get very hot? It escaped me at the time, but I do remember my C128 having some white smudge on top of the chips and the RF shield sporting flaps that rested on top of that. Later I read that this was supposed to conduct heat away from the chips into the shield, which was made of rather thin aluminium... I never felt any heat from the metal cover, so I don't think this worked very well, though.
Testing some heatsinks might be interesting indeed. And as you say, they would in the best case scenario remove heat from the chip faster, but it will still end up in the surrounding air. So to support the heat sinks you suggest people add a fan to their C64s?
I wonder how much airflow there really is, to begin with, inside the case of a C64.
Most of the heat of my C128 just rises through the top of the case that gets mildly warm.