I shall stand corrected, the 27% mana cost was my mistake, looked at the rank7 version, not rank9.
One of your initial questions was how Shadowburn dmg compares to SB.
Don't have access here at work to the new version simcraft, so I took the last one before the ptr (r1527) and modified the 55/0/16 spec to
54/0/17 to include shadowburn in the rotation.

Player=Warlock_T7_54_00_17 DPS=6035.8 (Error=+/-8.5 Range=+/-484) DPR=18.2 RPS=330.8/264.2 (mana)
Core Stats: strength=59 agility=61 stamina=1186 intellect=968 spirit=541 health=17995 mana=20231
Spell Stats: power=2000 hit=13.9% crit=16.8% penetration=0 haste=15.2% mp5=0
Attack Stats: power=737 hit=11.1% crit=12.4% expertise=0.0 penetration=0 haste=15.2%
Actions:
corruption Count= 1.1|84.7sec DPE=181369|13% DPET=136319 DPR=356.8 Miss=0.0% Tick=2316
curse_of_agony Count= 10.4|27.2sec DPE=11743| 7% DPET=10212 DPR=32.3 Miss=0.0% Tick=951
drain_soul Count= 6.9| 7.4sec DPE=31951|14% DPET=7081 DPR=62.9 Miss=0.0% Tick=14876
haunt Count= 21.1|13.0sec DPE=3494| 5% DPET=2723 DPR= 8.0 Miss=0.0% Hit=2927 Crit=4528|5827|35.4%
immolate Count= 13.8|15.8sec DPE=8717| 7% DPET=6688 DPR=13.3 Miss=0.1% Hit=1193 Crit=2498|2882|33.0% Tick=1522
pandemic Count= 37.9| 6.8sec DPE=1944| 5% DPET= 1 DPR= 1.$ Miss=0.0% Hit=1944
shadow_bolt Count= 55.0| 4.1sec DPE=7038|24% DPET=3509 DPR=10.7 Miss=0.1% Hit=4854 Crit=10142|13536|41.4%
shadow_burn Count= 16.2|16.7sec DPE=4361| 4% DPET=3404 DPR= 5.6 Miss=0.1% Hit=3135 Crit=6539|8286|36.1%
siphon_life Count= 8.7|32.2sec DPE=11465| 6% DPET=8939 DPR=19.7 Miss=0.0% Tick=1225
unstable_affliction Count= 17.6|15.4sec DPE=8145| 9% DPET=6346 DPR=15.0 Miss=0.0% Tick=1794
felhunter
felhunter_melee Count=173.4| 1.5sec DPE= 528| 6% DPET= 341 DPR= 1.$ Miss=0.0% Hit= 523 Crit=1046|1113|13.4% Glance= 392|23.9% Dodge=6.5%
shadow_bite Count= 44.3| 6.2sec DPE= 515| 1% DPET= 312 DPR= 1.$ Miss=0.0% Hit= 511 Crit=1024|1172|13.3% Glance= 384|24.0% Dodge=6.5%
Even if the numbers may be slightly off compared to the current ptr (especially the effect of ISB and the new Eradication), it can be a basis: it states that SB is higher DPCT, than Shadowburn - in fact Shadowburn has the lowest DPCT in the rotation (please point out if I made a mistake in the simulation, made it on the fly while working).
On the aftermath vs. shadowburn part: it would be interesting to see a simulation on that, however, for the +7dps from 1pt in Aftermath you don't have to sacrife a gcd, while including shadowburn in the rotation requires a gcd/cast. And as (if) SB is higher DPCT than shadowburn, you would lose dps using that time to cast shadowburn instead of SB. Alas, aftermath would be where that last point would need to go - given an ideal fight w/o moving (but simcraft is all about an indeal fight). In real raids shadowburn could have its use.
Although to make the question more complex, you would need to consider (besides the +7dps from 1pt in Aftermath) the dps gain if you would cast LT instead of Shadowburn (like in situations when you could cast Shadowburn, but couldn't cast any spell with cast time, or a dot due clipping). The spell power privided by the glyphed LT along with 4pt7 bonus could net you more dps if you refresh Haunt/UA/Immo/CoA in the 10sec(-gcd) time after casting LT.