I use a fully optimized COP set for timewalking. I pull generally 13k minimum sustained DPS, sometimes as high as 30k on aoe trash pulls, while dps who don't optimize their sets pull 2-3k by comparison. It's a sound investment if you have a group of people who want to steamroll these instances to farm the mounts.
But if you are serious about doing a lot of dmg in TW, you really don't have an option that isn't COP....sorry. You can't use AS to fast-clear mobs. AS is centered around padding your numbers on adds that are going to be alive for a long time. And the point of optimizing your dmg in timewalking is for them to NOT live a long time.
If any of your dps are decent, you probably aren't going to get more than 3 shadow word: pain ticks off on a high-health mob before the entire trash pack is dead, much less have any autistic spirits actually manage to reach their target.
When COP is combined with the shadow set bonuses from SoO it becomes deliciously OP. For AS, there are past-xpac set bonuses that boost the crit % of shadow word: pain, but the stats on these pieces are horrible (e.g. spirit on dps gear, which takes up the room a valid stat could have had) and they don't have as many sockets as the SoO gear, if any sockets at all -- which adds to the hopeless misery of attempting to play AS. Using gear with the maximum number of sockets is going to up your dps by a lot.
https://docs.google.com/spreadsheets...4UXgk/pubhtml#
This spreadsheet lists all the enchants and gems you should be using. For gems, it's EPIC gems only, no matter what xpac it came from. Pure-int is king, but use hybrid "reckless" gems if you have a good socket bonus to keep.
There are also advantages to using Warlords of Draenor items that "procced" a socket, rather than items that were "intended" to have a socket. WoWhead allows you to look up an item and check a box to see what stats it would have in Wotlk/BC/Cata timewalking respectively -- use it and check it out. Items with sockets generally have less static stats on them (i.e. less int on the item itself) to make up for the sockets, but it's still very very worth having the sockets regardless. You can loophole this, though, by using a WoD item that has an extra socket on it...because it gets the benefit of a socket without the drawback of having its other stats reduced.
PROTIP: Go for haste>mastery, not haste>multi. There isn't any multi on the old-world gear you will be using because it didn't exist until WoD, so don't bother trying to stack it. Even on aoe pulls, heavy mastery is godly when combined with COP and your SoO set bonuses...mind blast will be practically oneshotting the mobs.
My set is as follows...
(Note - the only way it could be better is if I had Ra-den's haste/mastery legs and used the SoO tier chest instead of tier legs, as I'd gain a socket that way. Sadly I can't solo past Durumu :P)
4pc SoO tier (Helm, Shoulder, Glove, Legs)
Neck: Socketed Gruul's Lip Ring (But you can use the haste/mastery socketed neck from Dragon Soul, or the JC blue boe crafted "Kuang's Locket of Secrets")
Cape: Legendary cloak (But you can use Kil'Jaeden's cloak if you don't have it. Despite being a lv70 cloak, WoWhead suggests it gives approx. the same stats as an appropriate-level piece in the wotlk and cata timewalking dungeons)
Chest: Robes of Riven Dreams (The only alternative I can think of for this is the chest off of Ra-den. It is much easier to buy infinite seals from the vendor and solo LFR Malkorok for these robes -- he is the first boss and you can queue up to kill him over and over and bonus roll infinitely.)
Bracer: Bracers of Unconquered Power (BoE crafted, Dragon Soul drop recipe. The only 2socketed bracers you can obtain.)
Belt: Orbital Belt from Elegon (But you can use the Garrosh belt)
Feet: Kavan's Forsaken Treads (Basically a freebie -- you can buy this for 249g off the Valor vendor in Stormwind/Orgrimmar)
Ring 1: Socketed Uk'urogg's corrupted seal (But you can use Thok's ring or Ring of the Riven from Dragon Soul)
Ring 2: ilvl715 epic ring from the legendary questline (if you didn't throw it out after you inexplicably got to keep it after getting the legendary)! The legendary ring is worthless, but the proc on this ring is a godly +10% intellect. The poor man's version of this would be the ICC exalted ring.
Trinket 1: Ghost Iron Dragonling with haste/haste/mastery cogwheels. The dragon doesn't attack, but the stats are worth it.
Trinket 2: Unblinking Gaze of Sethe, preferably a socketed version of any difficulty. This trinket is often on my top dmg done.
Weapon: Dragonwrath, Tarecgosa's Rest
Weapon alternatives: Sha-touched weapon set (if you have the legendary gem), Heroic Nibelung (from ICC), Garrosh heirloom staff (retired), or anything else with lots of sockets.
Glad to get that out of my system. Hope it was helpful
P.S. also worth noting that the legendary staff has some strange interaction with shadow word: death in WoD. Sometimes it lets you cast it only once when you should have been able to cast it a second time, sometimes it lets you cast it 3 or 4 times when it should have gone on cooldown, sometimes it gives you 2 orbs per cast AND lets you cast it 3 times or more.. I haven't been able to explain it. It does give you extra orbs from mind blast procs, at least. I know that much