The figure I've seen thrown around for mite drop on a SBB on a standard hive is 30%. Don't you think that this would be MUCH higher on a horizontal hive where there are not so many bees for the mites to catch on on the way down? I have a three box wide medium hive with a SBB that runs the length of it. I would think that gives me three times the chance of them not landing on a bee on the way down.
Of course by this estimate you would just have to make if FOUR boxes wide and you would get 120% of the mites. (can you say diminishing returns?)
Still I think it will help.