The spec would be this:

http://www.torhead.com/skill-calc#201ZfcM0dZfMfRs0krc.1

Ideology behind Rotation:

- Death Field has a 15 second CD
- Crushing Darkness has a 15 second CD, 2 second cast time.
- Force Lightning will be the main damaging spell.
- Affliction is up constantly.
- Shock + Chain Lightning are there are there for situational purposes.

ST Rotation:

Death Field > Affliction > Force Lightning > Crushing Darkness

Priority 1) Death Field used on CD
Priority 2) Affliction is always kept up
Priority 3) When 'Wrath' procs, Crushing Darkness is the next spell.
Priority 4) Force Lightning is used when all of the above are met.

Purpose of the Build:

Longevity:

Playing the standard Lightning build and regenerating Force via Subversion will give you an extra 2.4 force / sec regen (on top of the standard 8/sec). Playing this build, with Sith Efficacy, you regenerate an extra 6 force / sec regen (no top of the standard 8 sec).

This means that purely spamming Force Lightning would never cause you to lose Force (8/sec = 24, + 6/sec = 18 total 42 for a cost of 30). This build would allow you to go on forever while the standard Lightning build would ultimately, at some point, run out of energy or require the use of Consumption.

Damage:

Passive bonuses to damage for this build:
- +12% on Force Lightning
- Lightning Barrage
- +20% to all DoTs (Death Mark)
- +30% Critical damage to all DoTs (Creeping Death)
- +15% to Affliction
- +6 seconds to Affliction (40% more up time on DoT)
- +2 seconds to Crushing Dark (33% more up time on DoT)
- +6% damage to Crushing Darkness
- +20% damage to Crushing Darkness (cast under, and only under, Wrath)

Compared to the passive bonuses to damage for the standard Lightning build:
- +20% Alacrity every 2 minutes
- +6% damage to Lightning Strike, Thundering Blast, Chain Lightning, Crushing Darkness
- +6 seconds to Affliction (40% more up time on DoT)
- +50% Critical damage dealt by Affliction, Thundering Blast, Chain Lightning
- Lightning Barrage
- +3% damage to everything
- Forked Lightning

Looking at passive bonuses from a perspective of granting the most synergy, the Madness tree seems to do it far better.

Obviously it's hard to test because ... no combat logging ... but as I said, purely speculation? Can it work? I'm levelling a Sith Sorc as my third character (Level 50 BH Merc [healer] and Level 30 SW Marauder [Deeepz]) and would love to try damage from a peculiar perspective. I loved playing a Shadow Priest in WoW, and Madness mirrors it a lot.