For those of you tasked with killing Seige Engineers as Subtlety on Heroic Garrosh, I'll share what I've learned.
Out of stealth, open up with Garrote -> SnD -> Premed + Ambush -> Ambush -> Evis -> Backstab -> Evis
This will generally be enough to kill the first engineer if all of your trinkets are up and your engineer killing partner (or team) is worth their salt. If your engineer team struggles to get him down before his cast finishes, you can use shadow blades with your first ambush to ensure enough CP for another Backstab -> Evis. If this still isn't enough, consider shadow dancing and replacing each Backstab with Ambush (though it can lead to being energy starved as his cast is about to complete)
This is the only situation in this entire xpac (sans pvp) that I've found opening with Garrote to be advantageous for the subtlety opener. A single CP from garrote is more than enough time on SnD for the engineer. This minimizes the GCDs, Energy, and CP needed to get up a bleed effect, maximize Find Weakness, and start SnD.
If anybody else has a more effective way of killing the engineers as Subtlety, I'd more than welcome the information.