Note that I'm already getting very effective results out of spinning down my disks. I just take care which data I put on which disk. Spinning down the root volume effectively would be trickier, but doable.
By aggressive caching I meant preemptively reading big linear chunks of volume into host memory, a behaviour we plan to support after more pressing issues are under control. Spindown isn't our immediate priority by any means, but we can probably help make it practical in more situations.