Math is off. You are comparing ONE stack of Bloom with one penance…cept Penance is on a 12 second cooldown. So lets bring in the 3 stack Lifebloom for a true comparison since it would take roughly 12 seconds to get off 3 stacks and let it bloom.
So now we back to:
Penance 79 mana 420 total
LB 201 mana 528.
Penance is still superior tbh but you cant do a mana/power comparison unless you include ALL the factors.
Thats like comparing Homuncal Armor debuff to Warriors but only using one stack of the warriors of it (The rune has a XX second reapply to the armor debuff)