From my understanding, anything that invokes a GCD should still invoke a GCD if it's called, regardless of if it completed casting, came up instant or not. I don't use stopcasting because it's erratic and impacts my white DPS since I run melee.
Luckily I only have one toon now that needs a way to deal with casttime spells that proc instant {Maelstrom Weapon on my Enh Shaman}, and I use Randomized Casting to deal with it. Not as ideal as when I manually dealt with it, but it's a more than fair tradeoff for not having to watch it all the time.
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