There's no problem with doubling up on Piety's effect; DET already does the same as STR, INT, and MND, just to a lesser degree. Adding a TP regen boost to Skillspeed and an MP regen boost to Spellspeed are perfectly fine and would work much better (read: easier to calculate and implement) than trying to separate TP/MP regen tics from system tics. Regardless, I'm a PIE fiend with my WHM, so I'm already biased toward anything that increases my MP regen rates.