But the problem is that when a driver is mounted in a very small box, it starts to contain the heat. If the box is very small, it acts like the vent weren't there at all. So horns with small rear chambers are likely to suffer from heat related problems.

In a horn with a small rear chamber, there is less heat exchange because the back chamber air gets hot and can't dissipate. I imagine it gets pretty hot in there at full power, and then the vent doesn't provide much cooling. Some have used metal chamber panels in an effort to conduct heat to the surrounding air, but it seems to me that a better solution might be to just vent to outside air in the first place.

I guess the real test is to plumb the system for outside venting and see what happens to power handling. Then we'll know how much benefit it brings.