Guys on my gear i got straight 19 spell power gems , and three +32 spell power gems from JC.... i'm using Ember Skyflare Diamond so it can be active with the red gems.
I am at 2208 spell power with inner fire ( 2034 completely unbuffed) 21% crit with the talent and like 20hit rating undercaped which can easily be fixed with an item change.... so i propose full +19 spell power gems cause the difference between 21% and 22% crit for instance ain't as big as the difference between 2k and 2.1k spell power
Cheers