I have not really seen any math on the subject of Drain Soul, so I wanted to write something up, to take a stab at it. I am going to do this in a "Damage Per Tic" mode since DS is really focused around "tics" of damage :-) This also assumes your the only lock putting up affliction dots, since that is all you can count on.

Napkin Math incoming!

Drain Soul
Drains the soul of the target, causing 710 Shadow damage over 15 sec. If the target is at or below 25% health, Drain Soul causes four times the normal damage.

2k Spell Power assumed
40% Crit Rate

ie:
142 Shadow damage every 3 seconds
214.3% coefficent according to wowwiki

142 base damage + ((214.3% coefficient * 2000) / (15 sec duration / 3 second tic)) =
142 + ((2.143 * 2000) / 5) = 999 base Damage Per Tic (DpT).

Modifiers:
999 * 15% shadow mastery * 3% malediction * 13% earth and moon * 3% sanctified retribution (ret pally) * 10% shadow embrace =
999 * 1.15 * 1.03 * 1.13 * 1.03 * 1.10 = 1515 modified DpT.

Drain Soul Bonus < 25% Bonus
1515 * 4 = 6060 DS_DpT

So we have 6060 Drain Soul Damage per Tick.

Soul Siphon (rank 2):
Increases the amount drained by your Drain Life and Drain Soul spells by an additional 4% for each Affliction effect on the target, up to a maximum of 60% additional effect.

Affliction Effects: Corr, UA, CoA, SL, Haunt, SE, *DS = 7
(*DS: I believe DS counts as an Affliction Effect for the purpose of SS)

6060 * (Affliction Affects * 4%) =
6060 * 1.28 = 7756 SS_DS_DpT

7756 - 6060 = 1696 damage per tick boost from Soul Siphon or roughly 282 DpT per effect.

Numbers for Haunt: (Pretty sure I got something wrong here)
Haunt: 699 Base Damage + (42.68% coefficient) + (40% Crit Rating) * 15% shadow mastery * 3% malediction * 13% earth and moon * 3% sanctified retribution (ret pally) * 10% shadow embrace =
699 + (2000 * 1.4268) + 1.4 * 1.15 * 1.03 * 1.13 * 1.03 * 1.10 = 2684 DPC.

DOTS: (Taken from Fallens Post)
Corr (w/Pandemic): 12,046 DPC / 2409 DpT
UA (w/Pandemic): 8,103 DpC / 1620 DpT
CoA (Normalized & 3s Tics): 8516 DPC / 1064 DpT
Siphon Life: 6933 DpC / 693 DpT
Haunt: 2684 DpC

Well, the first question is with regards to Haunt. Since Haunt literally has a 10 second window, best case scenario is All Dots are refreshed and you do a Haunt->DS with 3 tics of DS (~10 seconds), then you would recast Haunt.

Haunt + Corr * 3 tics + UA * 3 tics + CoA * 3 tics + SL * 3 tics + DS * 3 tics + SS Bonus * 7 effects * 3 tics =
2684 + (2409 * 3) + (1620 * 3) + (1064 * 3) + (693 * 3) + (6060 * 3) + (282 * 7 * 3) =
2684 + 7227 + 4860 + 3192 + 2079 + 18180 + 5922 = 44,144 ~10 seconds = 4,414 DPS.

That's if all of your dots are already ticking, and you get to 3 ticks off the DS before having to refresh anything ... 4,414 seems pretty low considering that is max dps since you have to start refreshing dots after that first intial run, costing you DS time, ergo lowing your dps after that.

How is the math? I know I am not really accounting for haste or anything along that line, but just trying to get a feel for how much damage is really being added by each Dot.

Wrong. Contrary to haunt, SE boosts "periodic effects", not "damage over time effects", so it very well affects drains. You can easily test this on a target dummy.

You're absolutely right Morrigan. First stack of SE boosts DS by 1.09 and the second stack by 1.14. I tested that but didn't bother calculating the exact numbers, my bad.