Interesting findings i've done with spell speed for AST (No Sect).

At 60, apparently Spell Speed reduces the GCD after 26~27 points invested into AFTER the threshhold of 361 Spell Speed has been reached. Going from 360 to 361 Spell Speed takes the GCD from 2.5 to 2.49.

Going from 361 to 387 drops the GCD by 0.01

Going from 387 to 414 drops to GCD by 0.01

Going from 414 to 440 drops the GCD by 0.01

So on and so forth.

Equation: floor(New GCD) = 2.5 - (((Current Spell Speed - 334) / 26.5) * 0.01)