I;m not 100% sure, but I believe it doesn't, since the detonate just use the damage from the remaining DoT, and precision doesn't affect DoT.
Also precision specify it increase spell damage, not ultimate effect.
It is better for holy priest or hybrid, but it is the best talent in game I think only seasoned botanist is comparable to it. Even tho it's not as good for shadow priest it is still a really good talent for shadow priest , 40% extra damage on 2 spells, shadow bolt and dark frost, also make judgement get bonus damage from your mastery. It is still the best talent in game and is worth
However, thinking endgame-wise. two big things to think about "damage per second" that the ability adds and it's "damage per mana" cost. (my DPS numbers below are with +70% ability power, in thousands, the DPM numbers don't include ability power boosts, lvl 95 runes, and not counting critical hits or augment runes, all that stuff matters, but c'mon, who's got the time to type it all in a forum post)
There are what, 7 active rune slots on a weapon?
So, you slot shadow bolt, gloom, and shadow orbs - no question, these are great skills numbers-wise. High damage, not really high mana cost.
Shadow Bolt is so amazing, 1.3 dpm | 7.5 DPS
Gloom, nice as well, 0.5 dpm | 5.6 DPS
Shadow orbs - necessary to explode things - 0.26 dpm | 5.2 dps
That leaves 4 slots left.
Corrupting plague - high mana cost, longish cooldown, but nice debuff and good damage. <-- I slot this, I think it's worth it if you can afford the mana. (0.25 DPM, 3.5 dps)
Darkfrost - low damage (although... I don't have a number written down... maybe I should do some research!), medium cooldown, debuff not really that useful for endgame.
Edit: got a darkfrost lvl 95 rune, dunno if it's tops, but 11k. Added it to spreadsheet and info here: 0.25 DPM | 2.4 DPS , goes to 3 DPS with precision.
Vampiric Pact - boosts will - no thanks. Will, unfortunately, is poorly designed. Maximum boost of 8 mana per second in combat. That is tiny. (and that's maximum, with diminishing returns, you'll never make it there). At end game, don't slot this.
Shadow blast - It's AoE, biiig mana cost, not a ton of damage, but hey, it's AoE. (damage/mana is about 0.14), medium cooldown, too. (0.14 DPM, 2.4 DPS)
So, take those options and compare to adding holy bolt or smite, even with no investment whatsoever in holy mastery (so, 0% ability power bonus)
Holy bolt- (0.19 DPM | 2.9 DPS)
Smite - (0.3 DPM | 2.4 DPs)
So, what are you gonna slot? Even if you slot shadow blast and corrupting plague (fine choices!), you then have 2 slots left, even if you slot darkfrost, you can still slot one of these for more DPS instead of vampiric pact. AND, it's pretty quick return on investment for one or two pieces of holy mastery to get up to +10% holy power (plus any bonuses from flux, poise, glory, mastery dram, gralla seed, ash blight boots), so even though you aren't specced holy, these holy abilities can provide some benefit.
Plus.... if you add in precision, you boost Shadow orbs by 1.3 DPS, holy bolt +1.2 DPS, smite +1.2 DPS.
If you are going pure damage (not slotting any healing or doing AoE cure spell in holy talent tree), then precision is the way to go. You can afford it (at end game mind you!)
With the Talent Builder you can plan out your perfect build, and share it with others!
This is lvl 95, no rebirths, pure damage talents. Vile echoes nets you some mana regen with shadow weave ultimate.
Oddly, to Verrier's point about "hybrid being the most damage", I completely agree skills runes-wise. I don't think hybrid talents are needed for a maximum damage build (like the above). (although, if you did that this build to more rebirths, I guess you probably would put the additional points into final holy ability or maybe burden of power.)
If I find a lvl 95 darkfrost rune, I'll update that in the post.