What the heck.. I've just ordered the HD27 from Costco so that I can do a head-to-head comparison of the two, and also to see if the HD27 is quieter or not.

The BenQ is a lot quieter than the ABOX A6, but is still louder than I would like for a projector that is sitting more or less directly over my head. The Optoma HD27 has a larger vertical offset, so I should be able to mount it another 6-7 inches higher up, which in-theory would reduce perceived noise levels. We'll see.