I think that you're probably right that the voice coil heats pretty quickly after power is applied. But I also think that anything that removes heat will help. I imagine that the whole idea of venting was to carry some of the heat away, and as inefficient as it is, I expect it probably provides some benefit. I also imagine that if the pumping action of the vent were improved, the thermal performance would also be improved, compression reduced and power handling increased.

What say you?