Consider melding Det over DH for prog, as the damage benefits are basically equal on average. Meld SpS to a comfy tier if you wish, low priority during prog.
Piety is a binary stat, as any more than you needed to not run out of mp was wasted stats. Because of this, when choosing our substats, avoiding excess Piety is the best way for us to increase our damage, and you’ll sometimes see our low piety sets use lower iLvl equipment to avoid Piety (this is still the exception rather than the rule, don’t actively use lower iLvl items to dodge Piety without calculating out the gain yourself). It’s hard to predict exactly how much Piety you might need in most settings, but you can take a look at how much the recommended sets take in each setting and decide which one fits you best.
Remember that slower GCD tiers use significantly less mana (one less Glare per minute is 400MP saved per minute) and can thus get away with less Piety. If you find yourself struggling with MP, ensure that you’re effectively using your mp tools before adding more Piety. Swapping GCD heals for OGCD heals, using Assize and Lucid Dreaming on CD, using Recitation for a free GCD heals in a pinch: these are all effective ways to make your MP go farther. Also make sure to always carry HQ Super-Ethers when raiding, they are a very effective way to recover from multiple rezzes or a death.
You can find stat tiers here: http://allaganstudies.akhmorning.com/stats/