Ty mate
I didn't really test it since new core i never raided as hybrid. But if i go from demo spec point of view because both specs have the same amount haste, i love the way demo preforms with high level of haste (by that i mean 1745+ with DFO i love 1,4 sec cast on soul fire and 1.5 on shadow bolt). Specially on bosses where you have big decimation up time like PP, BPQ, LK, Halion even DBS, on this bosses for sure i can tell you that Spellstone is out preforming Firestone with a lot.The only way Firestone can be better is if fights are quick and you have good RNG on crit and Molten Core. But on the other hand imagine how much better will be faster casts with good RNG on crit. Because we take in mind just the pure crit that stone give as that 1% damage on direct spell more is nothing. Even if you can keep 20k dps that is 200 dps increase, and its not even that because not 100% from your damage is direct you have Corruption, COA, COD, Immolation Dot.
Even just on quick look on the stats Spellstone looks a lot better;]/:
Spellstone 240 haste
Firestone 196 crit
But test it yourself and tell me if you see Firestone preforms better. Keep in mind that im compering both on my point of view with almost bis warlock.