Not to throw a wrench into the FFB discussion, but I realized something while working on something for frost mage PvP on the main mage forums (
WoW Forums -> "Double" Shatter Combo @ 96 Haste (VIDEO)) that might be of interest regarding WG.
Lhivera, Vontre, et all doing the modeling for this, its worth considering that while you're effectively doing shatter combos under WG using Frostbolt->Ice Lance combos on procs, the timeline of casting will allow the ice lance to be potentially significantly shorter than the 1.5 second expected time since the GCD will start client side as soon as the spell cast request is sent, not when the spell cast actually happens. I've been able to observe this up to 300 msec from the end of the frostbolt cast (meaning the Ice Lances 1.5 second effective cast time really was 1.2 seconds for all intents and purposes).
What the average return on it is I'm not really certain, and will be incredibly variable depending on latency and such, but compressing the DPS cycle by sneaking that GCD in early could theoretically result in an unrealized gain compared to perfect world modeling.