I read the same thing as Baalz, that's why I was wondering if spell boosting during battle would work for those paths that exist on the tactical level (i.e. battle) , but not strategic level (i.e. map).
The communicants can only be communion slaves. The crystal matrixes are to make mages unskilled in astral magic (illusionists and wizards) become communion masters. Astral mages can cast "communion master", but others can't.
Actually, I tried using these items and it did not work. I put a master matrix (or whatever it is called) on an Arco Astrologer. I gave slaves matrices to two commanders. The Astrologer's magic stats did not go up. I checked during battle.
I then tried the same thing using two more astrologers with the slaves. No boost.
What am I doing wrong? I ask because I really wanted to use these magic items in this game. I was going to experiment with it this time around.
Communion path boosts don't show up in the unit info IIRC, but should still be in effect. If you want to test it for sure try casting something you couldn't cast normally and see if it works.