I've been playing around with the simulator and min-maxing the gear and specs. With the set bonuses the way they are and the itemisation of TotC gear, you can put together a set that is very well optimised for unholy dps. Taking this as a baseline and playing around with specs, I get a DW epidemic Glyph of Disease based spec as simulating the best in item-level 245 gear.
Looking at the numbers this is due to the exceptional scaling on the diseases, and the fact that the standard spec ends up losing many ticks due to clipping. Assuming you've got maxed diseases then the basic strategy is pretty simple keep up diseases, desolation and bone shield, and use ScourgeStrike, Blood Boil and Horn to generate runic power and Death Coil to dump it.
I haven't worked through the details of the exact best rotation or priority, but using a priority of
diseases>MaxRP_DC>BoneShield>SS>BloodStrike>DC with a 1/13/57 (shift 2 points in subversion to epidemic)
I get
9281 DPS with the following breakdown (it actually sims better with the short fight option - 9310 DPS)

Ability Total % Landed Hit% Crit% Miss% Average
MainHand 473349156 14.2 165476 49.8 36.3 13.9 2860.5
Ghoul 402378022 12 340199 87 13 0 1182.8
ScourgeStrike 375573168 11.2 69080 52.4 47.6 0 5436.8
DeathCoil 362922850 10.9 53264 67.7 32.3 0 6813.7
BloodPlague 315899139 9.5 119737 63.8 36.2 0 2638.3
FrostFever 313334861 9.4 119711 67.7 32.3 0 2617.4
OffHand 271474017 8.1 285936 49.9 36.2 13.9 949.4
Wandering Plague 225747219 6.8 85942 100 0 0 2626.7
Gargoyle 169961570 5.1 35341 87.2 12.8 0 4809.2
Necrosis 148955102 4.5 451412 100 0 0 330
BloodCakedBlade 119281935 3.6 136003 100 0 0 877.1
BloodStrike 87073358 2.6 36186 58.7 41.3 0 2406.3
UnholyBlight 72589246 2.2 53264 100 0 0 1362.8
IcyTouch 1396949 0 354 67.2 32.8 0 3946.2
PlagueStrike 1164772 0 425 59.1 40.9 0 2740.6
Horn 0 0 16853 100 0 0 0
Pestilence 0 0 24191 100 0 0 0
BoneShield 0 0 5942 100 0 0 0
BloodTap 0 0 2344 100 0 0 0
DPS 9281
Total Damage 3341.1m in 100h
Threat Per Second 4660
Generated in 555s
Template: disease
Priority: disease
Presence: Blood
Sigil: Virulence
RuneEnchant: FallenCrusader / FallenCrusader
Pet Calculation: True
Whereas using a 20 second PS IT SS BS BB DC DC PS IT SS BB BB DC DC rotation with the standard 3/13/55
I get
9072 DPS

Ability Total % Landed Hit% Crit% Miss% Average
MainHand 470221372 14.4 165720 50 36.3 13.8 2837.4
DeathCoil 406161921 12.4 59736 67.7 32.3 0 6799.3
Ghoul 400410584 12.3 340199 87 13 0 1177
OffHand 269323656 8.2 285690 49.8 36.2 14 942.7
BloodPlague 239056705 7.3 98141 63.5 36.5 0 2435.8
FrostFever 230960645 7.1 98150 67.7 32.3 0 2353.1
ScourgeStrike 176574685 5.4 32216 48.4 51.6 0 5481
Gargoyle 173439580 5.3 36064 87 13 0 4809.2
Wandering Plague 171114452 5.2 71443 100 0 0 2395.1
Necrosis 147902867 4.5 451410 100 0 0 327.6
Blood Boil 133734718 4.1 42955 67.8 32.2 0 3113.4
IcyTouch 120994195 3.7 32217 68.2 31.8 0 3755.6
BloodCakedBlade 118939907 3.6 136002 100 0 0 874.5
PlagueStrike 88178969 2.7 32217 57.5 42.5 0 2737
UnholyBlight 81237525 2.5 59736 100 0 0 1359.9
BloodStrike 37732840 1.2 16108 54.5 45.5 0 2342.5
Horn 0 0 4230 100 0 0 0
BoneShield 0 0 5369 100 0 0 0
BloodTap 0 0 5275 100 0 0 0
DPS 9072
Total Damage 3265.98m in 100h
Threat Per Second 4528
Generated in 202s
Template: unholyDW
Rotation: unholyDW
Presence: Blood
Sigil: Virulence
RuneEnchant: FallenCrusader / FallenCrusader
Pet Calculation: True
The stats for this was from a gear set i put together of ilevel 245 with a few tweaks. but i don't think it's unrealistic

<?xml version="1.0"?>
<character>
<stat>
<Strength>2453</Strength>
<Agility>156</Agility>
<Intel>58</Intel>
<Armor>14696</Armor>
<AttackPower>671</AttackPower>
<HitRating>334</HitRating>
<CritRating>842</CritRating>
<HasteRating>349</HasteRating>
<ArmorPenetrationRating>199</ArmorPenetrationRating>
<ExpertiseRating>177</ExpertiseRating>
</stat>
<weapon>
<count>2</count>
<mainhand>
<dps>196.5</dps>
<speed>2.6</speed>
</mainhand>
<offhand>
<dps>196.5</dps>
<speed>1.5</speed>
</offhand>
</weapon>
<EP><base>50</base></EP>
<Set>
<T72PDPS>0</T72PDPS>
<T74PDPS>0</T74PDPS>
<T82PDPS>0</T82PDPS>
<T84PDPS>0</T84PDPS>
<T92PDPS>1</T92PDPS>
<T94PDPS>1</T94PDPS>
<T72PTNK>0</T72PTNK>
<T74PTNK>0</T74PTNK>
<T82PTNK>0</T82PTNK>
<T84PTNK>0</T84PTNK>
</Set>
<ChaoticSkyflareDiamond>1</ChaoticSkyflareDiamond>
<trinket>
<MjolnirRunestone>0</MjolnirRunestone>
<GrimToll>0</GrimToll>
<BitterAnguish>0</BitterAnguish>
<Mirror>0</Mirror>
<Greatness>0</Greatness>
<DCDeath>0</DCDeath>
<Victory>0</Victory>
<Necromantic>0</Necromantic>
<Bandit>0</Bandit>
<Pyrite>0</Pyrite>
<DarkMatter>0</DarkMatter>
<OldGod>0</OldGod>
<Comet>0</Comet>
<DeathChoice>1</DeathChoice>
<DeathChoiceHeroic>0</DeathChoiceHeroic>
</trinket>
</character>
With this build the glyph of dark disease and glyph of the ghoul simulate near each other in value and the new icy touch glyph comes out to be about 1.5 times better.
A quick EP of the main stats:
EP:50 | AttackPower | 1
EP:50 | Strength | 3.06
EP:50 | CritRating | 2.58
EP:50 | HasteRating | 2.58
EP:50 | ArmorPenetrationRating | 1.15
EP:50 | ExpertiseRating | 1.45
I like this build better because it's less dependent on haste and less latency sensitive. the play-style should be a lot freer, and of course the multi-target damage will be better.