The way it reads in the Comapnion (just checked it again last night) is that you "use" your POW to gain Divine Magic, so your effective POW goes down.
When you cast or release the Divine Spell, the POW used in the spell comes back and adds to your real POW.
So, yes, it could increase your POW past 21, say, if you had POW gain rolls in the meantime. It also means that people with a lot of Divine Magic can release their spells, 1 combat action/round per spell, and massively increase POW.
Clearly, that's not the way it's meant to work, no matter what the rules say.
I would play that it frees up Dedicated POW to be used again, rather than increasing your actual POW. This has several benefits:
1. It's not silly!
2. It doesn't allow priests to massively increase their POW in combat.
3. It allows priests to build up a large store of Divine Magic.
4. It makes Divine Magic useful again.
Now, the fact that Divine Magic is gained in chunks was a little odd for me. If I get Shield 4 then I can't top it up with another Shield2 until I cast the Shield 4. That seemed odd. But, there is a workaround - when you go to the temple, you release the Shield 4 and sacrifice for Shield 6, using 4 points of your Dedicated POW and 2 points of POW. You still gain Shield 6 at the same cost and have got around the silly rule. It almost makes it pointless having that rule in the first place.
Of course, it can only be done at a largish temple, whereas getting Shield 2 can be done at a small temple, so it does restrict where you can gain larger chunks of spells.
I don't have Companion with me, but there is a Divine Spell which allows you to bless a Holy Place (Sanctify/Consecrate? I can't remember the name of the spell). All well and good. It is progressive and you can cast it at high Magnitudes, to make a spot holier. Fine. In fact, you can gain spells of equal magnitude to the spell as if you were in a temple, so if I cast Consecrate/Sancity/whatever-it's-called 10 then people can gain up to 10 magnitude spells in the place - great. But, the spell is a normal divine spell (I think) and you get the points back when you cast it, so what's to stop you doing this again and again and again? I would make it one-use so that you lose the POW when you cast it.
Otherwise, I cast Consecrate 10, get the Dedicated POW back, get Consecrate 10 back, go somewhere else and cast it again. You see what I mean?
Hopefully, I've misread or misremembered the spell and it is one-use.
Apart from that, the Divine Magic rules aren't bad at all. Truesword is a Magnitude 3 spell, which makes it expensive. Resurrect is Magnitude 5, which is probably better than a 3 point spell. I like the idea that Regrow Limb allows you to reattach a limb if you have the limb and that it takes a set time, which is quick, to regrow a limb. Of course, a DEX30 healer regrows a limb faster than a DEX 5 healer, which is a bit odd, not bad but odd.
Getting the POW back when you cast the spell means that Divine Magic is more flexible than it used to be - you can get spells depending on the forthcoming mission. So, a Chalana Arroy priestess going after broos can put her spells into Cure Disease spells, then when she comes back and goes against ogres, she can get Heal Body spells instead. That is an improvement.
There are three sample cults in the Companion, none of which have divine magic listed in the cult. That is odd indeed. I know the spells have the type of cult that has them (Reflection - Trickster, Absorption - Night, Earth and so on) but I would have thought it would have been better to list sample divine spells for each of the sample cults.