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!
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
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).
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.
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.