Soft cap to get GCD of all spells to 1 second is
50% haste.. In a raid environment with moonkin aura/totem and swift retribution, it comes down to 38.7% haste.. (equivalent of 1269 rating with spellstone on)
During normal phase,
- shadowbolt - 1.7 seconds without BL and 1.3 seconds with BL
- molten-cored incinerate - 1.17 seconds without BL and gets GCD capped during BL - 1 second..
During decimation phase,
- soulfire - 1.6 seconds without hero/BL and 1.2 seconds with it
As you can see, only one of your main nukes becomes GCD capped (and that during hero/BL only)..
Stacking haste beyond this point isnt such a bad idea TBH, as it will still lower the cast time of all our fillers..
If you want to switch out spellstone for firestone, make sure you get 1269 haste without spellstone.. At that point firestone might be more dps since it boosts soulfire, incinerate and SB by 1%, not to mention the ~5% bonus crit