See, and this is the stupidest part, you're TECHNICALLY correct, but you're PRACTICALLY wrong, because the increased amount of Syphons+Bloodspillers you'd be gaining with an increased GCD are disproportionally wasted because of current DRK MP economies. Even at the fastest possible post rework DRK speeds (specifcally, max Haste Bozja gear), it can drop your GCDs down to 2.10~ if I recall, but it still doesn't effectively matter because at our slow set, 2.5GCD BiS (25 GCDs per minute), compared to a theoretical 2.10 set (note, which is not possible even with max SkS gear and melds), would only give you around 28 GCDs per minute, adding in one extra combo, for simplicities sake.
You can then extrapolate that over the encounter, take the amount of combos you'd need to generate an extra Edge of Shadow which should take around 5 minutes for a single extra, weaker Edge of Shadow that'll probably have it's value lessened by a drastically inferior critical hit stat (being that it is rate+damage), and no direct hit. Also note that this is a bit inaccuruate, because an extra Bloodspiller is gained every 3 minutes, that combo string is delayed. Additionally, with Bloodspiller again having the same innate attribute being relatively high potency, in my opinion, there's less overall value since Bloodspillers are not only gained via basic rotation competence, but also just given freely every minute, it brings quality of each individual Bloodspiller down by forcing SkS.
I would meld SkS, because I find it more fun, but it's the overwhelming superiority of Critical Hit and Direct Hit, particularly in high burst damage, raid buff synchronized compositions AKA DRK's primary offensive niche that necessitate it. It's not just that it's not a BiS stat, SkS is a borderline useless stat outside of comfort/minimum thresholds, kinda like Accuracy, that is mitigated even further by the recent BW change, because it takes too much away from DRK's potential burst ceiling. To not abuse it is tantamount to griefing. I don't like this. Increasing a 12K MP per minute MP economy to a 12.6K to 13K MP per minute economy just isn't effective, and you can attribute some of that to passive MP regeneration as well, lessening the impact.
AND that's without getting into the Dark Knight classic, where Skill Speed doesn't work on Unmend, Unleash, or Stalwart Soul, gives you a dead stat for your ranged ability that Enhanced Unmend doesn't make up for, and 2/3rds of your GCD AoE rotation.
Believe me, if I could run a 1.98 GCD again (Thanks Haste BW/Eureka!), I would.