I checked a unit that has a Burning Blade spell on it and it is only adding 10 damage with 4 fire shards plus a fire temple, it should be 12 (the spell is 2 + 2 per fire shard), so there's no virtual fire shard thing that's being added.

I went and looked at CoreImprovements.xml and compared the Fire Temple to Ereog's Tower (which DOES provide +1 air power), and it should work, the only difference is that Ereog's Tower has  <UpgradeComparisonID>1</UpgradeComparisonID> and the Fire Temple has  <UpgradeComparisonID>2</UpgradeComparisonID> 

I also checked and the Air Temple doesn't work either. It seems the only difference is in the UpgradeComparisonID.

Fire Temple:

        <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>FireMana</Attribute>
            <Value>1</Value>
            <PerTurn>1</PerTurn>
            <Provides>+1 Fire Power</Provides>
            <UpgradeComparisonID>2</UpgradeComparisonID>
        </GameModifier>

Ereog's Tower:

        <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>AirMana</Attribute>
            <Value>1</Value>
            <PerTurn>1</PerTurn>
            <Provides>+1 Air Power</Provides>
            <UpgradeComparisonID>1</UpgradeComparisonID>
        </GameModifier>

Air Temple

        <GameModifier>
            <ModType>Resource</ModType>
            <Attribute>AirMana</Attribute>
            <Value>1</Value>
            <PerTurn>1</PerTurn>
            <Provides>+1 Air Power</Provides>
            <UpgradeComparisonID>2</UpgradeComparisonID>
        </GameModifier>

I'm stumped.

Well that's a bummer. I'd read in another post about the temples increasing your shard power, so I excitedly started another game and pursued a line of technology that I'd never used before. Those are some seriously expensive buildings for a bit more mana. By the time you can get them, I'm swimming in so much mana that I don't really need the increase, but I could sure use a boost to my shards. I suppose it would be easy enough to mod by just changing that value after the AirMana part.

I think its confusing because of the notation

+1 Shard -> +1 Shard

+1 Mana -> +2 Mana

Unless you don't closely look at it you wont realize that only the mana is going up. 

Value shouldn't change from what I can tell. The only thing that should change is the +2 Fire power if I am understanding things correctly. If you wanted to add more mana the PerTurn would change I think.

The other mods I've done have involved making bigger maps and removing level requirements for weapons/armor, and that was just modding files and dropping them in the mod folder. Apparently to modify these temples I had to turn on the localdataonly switch and modifying the file inside the Fallen Enchantress folder itself. So yeah, it works just as I suspected, modifying that value. Awesome!

Make backups of the original file(s)! You will not be able to update the game if you have changed anything in the original files.