I play an non-goblin shadow priest. My gear is currently at ilvl ~565. My average ingame latency is always 8-10ms. I had my shadow reforged to maximum haste, which resulted in around 16400 haste. My gear didn't allow me to get any higher.
I was always able to cast Mind Blast or Vampiric Touch and when the cast bar had ~0,3s left cast some other spell, both instant casts and spells with casttime worked.
Last week I finally got the Garrosh haste trinket, so I was able to reach my hastecap. The best I could get from reforging was 18347 (50.33%), my Mind Blast casttime shows 0.998s in the tooltip.
This week I raided with 50.33% haste and noticed that many times when I cast Mind Blast or Vampiric Touch and try to cast something else during the casttime I will get an "action not ready" error.
Do any other shadow priests also have this problem?
My guess is that WoW tries to execute the 2nd spell directly after the first one has been cast, but since the first one only took 0.998s the game enforces the 1.0s GCD cap?
Would it be better to go below the hastecap and never a single point above? The best reforge I can get below cap is 17958.