Agreed that they'll all be running cooler now, especially the middle drive. I specifically recall cooking a middle drive in that kind of layout once before.

But remember you're dealing with a case design and bay spacing which was originally meant for IBM PCs with an old, slow, cool-running HDD in one bay and a 5.5" floppy drive in the other bay. Later, that bay system was mostly intended to combine 1-2 HDDs, a CD-ROM, and a floppy. The design of the spacing of the bays isn't really optimal for a giant stack of modern high speed hard disks which put out a lot of heat. What you did, to give them better spacing, was very wise.

Most NAS units I've seen, though, include enough space in their default design to ventilate the drives and get the fan air moving past them. Though I haven't looked at a Drobo close-up: If the Drobo isn't doing that job sufficiently, then I agree that's a problem.