Unless someone pipes up with more information, here's a few things to keep in mind:
- The GCD without Haste is 1.5 seconds
- With 50% Haste the GCD is 1 second
- With 100% Haste the GCD is 0.5 seconds
- The GCD won't drop below 1 second
- More than 50% Haste will drop a spell below 1 second
That means that if you have enough Haste to make a spell cast or refresh in 0.75 seconds, you will still have to wait 0.25 seconds before you can cast it again because of the GCD cap of 1 second.
Does that help?