I have not used one, so take this for what it is. The vortex type separators are designed to separate out the larger dust particles and chips from the air stream. The smaller, lighter dust particles that remain suspended in the air won't drop into the separator bucket, but continue on to your vac. Since drywall dust is pretty much all fine particles, I don't think it will have much of an effect.
I do use a larger one in my shop, and they work quite well for dust and chips from my planer and jointer, but I can tell when emptying the can that the contents are quite coarse, where what drops into the bag of the DC is quite fine.
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