This is an excellent question. My carb heat was very ineffective, so two years ago I completely re-did the air intake. One of my concerns was ram-air from my Sam James air intake over-powering the low pressure air intake under the cowl. I thought about a pressurized intake for cab-heat, but steered away for a few reasons:
1. It is an un-filtered intake, so for the 99.999% of the time that I am not using carb heat I don't want the chance of un-filtered air sneaking past the filtered air.
2. I wanted the simplest operation possible so that when I need carb heat I know it is going to work. I did not want an extra door in the air path, but without one I would have ram air entering the cowl all of the time causing extra drag, and reducing the effectiveness of the pressure drop that I need for my oil cooler.
3. I found that with some good thought and a little effort I was able to fabricate a door that very effectively blocks-off the ram air from the air intake when I have carb heat selected.
Before my work I had a 15 degree carb temperature increase at altitude and speed. After the work, which included a new carb heat muff, I am seeing 80 degrees.