I don't seem to recall anyone making this point, but it seems to be that a spirit provides a larger multiplier for not only being of the same rarity as the units it's being fused with, but also the same rarity of that which it is being fused into. I was noticing that gold spirits were more effective as a boost to my gold units than silver spirits, despite the fact that I was fusing them with copper units, so they -should- have been the same. But I don't have enough spirits to test out this on other rarities anymore.

Most notably I'm curious about Platinum Armors. Are they best used on platinum units then? Can someone confirm this? A Plat armor should -normally- be worth 8000, but on a plat unit is it a higher multiplier?