Got curious about how much MP per tick Piety was actually giving.

The answer is .. Not a whole lot.
Base MP regen across all MP jobs is 200 per tick.
Naked with base Piety of 340, SCH gets 200 MP per tick.
Adding 120 Piety to that changes the tick to 205 at 460 Piety.
At 2286 Piety, I get 288 per tick. Adding 120 Piety to bring me to 2406 improves that to 293.
Adding 2066 Piety netted me a grand increase of just 93 MP per tick.

So it seems no matter how high or low our Piety is, 120 Piety will return 5 MP per tick.

Is this the intended scaling for Piety, or did they just lowball it with a final adjustment when Eden launches?