The amount a creature's power and toughness are reduced is determined only once, at the time Phyresis Outbreak resolves
With several cards that care about the number of face-down permanents you control, you'll be manifesting your victory in no time
On the other hand, if several of your creatures die at the same time because someone casts a board wipe like Wrath of God, each of your creatures triggers the recover ability of cards already in your graveyard
This leads to some interesting discoveries
While a very efficient tutor for the mana cost, sacrificing a creature was a pretty big ask when the Commander format was slower, running more akin to a battlecruiser environment full of big creatures slugging it out than the much more intricate version we play today
While it doesnt increase mana production from mana rocks like Kinnan, Bonder Prodigy, it does help make up for this by turning one of your lands into a creature that can now produce extra mana