Hope this helps some? I know it doesn't address your question directly.
For those with past SSD purchases, the bad news is that the DEVSLP feature requires both a hardware and firmware change in the build of an SSD. All but the most recent SSDs are not compatible and the only consumer SSD that we know of right now is the Plextor m5M mSATA SSD.
Thanks for that, but as nothing yet has been bought, indeed, with the D54250WYK being reviewed over a month ago, and numerous vendors listing it, I had assumed it was available to buy, so I had only a few queries like a list of testes compatible SSDs and the above before pulling the trigger. As the 530 Intel fitted to all review models is still not yet for sale, I had researched the few low 'Idle' power alternatives, like Plextor M5M & Sandisk X110, and it seem all recent mSATAs I have seen announced since last CES have DEVSLP in their specs, which is another reason I was surprised not to see it in the Haswell NUC spec.
The Haswell NUCs do not support the DEVSLP feature.
DEVSLP is a function to allow mSATA SSDs to enter a lower power state than typical Link Power Management will allow, potentially giving you about 0.05 W of power saving. This is of much more value to a battery powered system.
DEVSLP would have required an additional signal connected to the mSATA connector; but that signal is not connected on the Haswell NUCs. We have measured low power configurations of the Haswell NUCs at approximately 4.5W; an additional 0.05W reduction would be barely measurable.