Is that most efficient? As in: would a larger board not be better? Yes, it may cost more (even per mhash), but there is a not insignificant overhead to "host" the boards. Tiven the standard MSI / Sempron approach I think 5 or maybe 6 boards only could go on a motherboard. Having a higher density, especially given the low power consumption, would not be negative.
Then there is the whole driver issue at hand (sadly)

Lokos like I would have to move to Linux.
Why have servers at all? For a couple of dollars more, you could equip those boards with an ethernet port and a small ARM processor running linux, with a ready-to-go firmware preinstalled, which would be configured through a web interface. Or possibly a backplane with the ARM processor and ethernet, which has a couple of slots for crypto slave boards containing the ASICs.