the fairy buff lowers my global from 2.44 to 2.33
i have no idea what number it could possibly be multiplying by 30% to make that happen... what seems to be going on is that it's tripling your current gcd reduction from gear (best guess)
greased lightning actually does what it says it does, reduces your global by 5% per stack (although it uses your global after gear is taken into account.. meaning it scales inversely with skillspeed)
tldr - fairy buff is way way way worse than advertised. (in my case .11/2.44 is like 4.5%)