Not so much a necro here, despite the delay and the theme of the Elsweyr chapter, but the new patch notes for that chapter do indeed say that the cast time skills presently have an extra 200ms cooldown on top of the GCD, so it seems that @twofaced was right in his math.

"Removed the 200ms post global cooldown from cast time abilities. This was done to improve the feeling of using these abilities in rapid succession with other abilities, to allow for a more fluid transition, and less down time between usage. We also made some adjustments to the cast times and power levels of specific abilities, which will be called out on a per case basis in further notes."