How about Hydros? Adds in Hydross have more hp and it takes longer to kill them... Do your shadowpriests chance to adds odn Hydross?
I usaly dot up all non-banished adds then help them burn down one or possibly two adds really fast before going full time on hydross. The idea is that it's worth it to lose a bit of efficiency for the sake of reducing incoming damage but after 1-2 adds are dead there's spare tanks and healers so it's not really a problem.
It's worth mentioning that we're cheesing it with the 250% nature phase all the time. If we made the switches as intended there's no realistic way that I could spare helping on the adds every switch, both personaly and for the other dps casters and healers I'm feeding.
The short answer is that shadowpriests helping on adds is a desperate measure that should be avoided if possible due to the fact that your shadowpriest will go oom and stop doing damage and feeding his group with mana and hp. No other class ingame suffers so severly from switching to adds, fire mages is probably nr2 on that list and they still lose far far less.
caveat: The exception to this is threat sensitive situations, in some cases dps'ing down multiple mobs at the same time gives the mt a chance to build up enough threat so I can go full out after that. my best example is probably magtheridons adds where i start off by spreading out damage over two adds for a short while before hard focus on one.
Some classes need to stay on their target or they lose a significant amount of DPS invested. Shadow Priests need VE, VT, Misery, 5x Shadow Weaving which takes them at bare minimum 7 GCDs to build up to.
Hunters need to cast Hunter's Mark and perhaps a sting, but neither of which decreases their DPS by 20% (the loss of Misery and 5x Shadow Weaving). In addition, without VT and VE the spriest is just a sup-par warlock.
Mages lose scorch debuffs, rogues lose combo points, warriors lose sunder, etc. Everyone loses something for target switching, but it really really hits spriests hard.
Some classes need to stay on their target or they lose a significant amount of DPS invested. Shadow Priests need VE, VT, Misery, 5x Shadow Weaving which takes them at bare minimum 7 GCDs to build up to.
Hunters need to cast Hunter's Mark and perhaps a sting, but neither of which decreases their DPS by 20% (the loss of Misery and 5x Shadow Weaving). In addition, without VT and VE the spriest is just a sup-par warlock.
Mages lose scorch debuffs, rogues lose combo points, warriors lose sunder, etc. Everyone loses something for target switching, but it really really hits spriests hard.
I'd argue that shadow priests only need Vampiric Touch and 3x Shadow Weaving to be reasonably effective. I've actually stopped using Embrace in most raid situations just to reduce my threat. The amount of mana regen I give healers is more than enough for them to our group anyway, and that's a better distribution of threat. And while Misery is "required", it doesn't cost a cooldown. It's also worth noting that two shadow priests DPSing an add is better than one because you'll stack up to 5x weaving in under 5 seconds.
That said, I absolutely agree that rapid target switching (at least once every 10 seconds) hits shadow priests the hardest. I generally don't switch off the boss to work on adds unless A) the raid really needs the DPS, B) it will take at least 12 seconds to kill (4/5ths of Vampiric Touch), and C) I can toss a random spell on the boss to keep up the 5x Weaving stack.
I agree with all of you that mana efficiency is a huge reason to not switch adds. However, on Curator mana efficiency doesn't matter...I don't see how you can run out of mana on that fight. The OP should not consider mana efficiency a factor for one of the encounters he specifically listed.
A few have also said that Shadow Priests take 2+ seconds to start dps on an add. It shouldn't take that long to /assist, /cast Mind Flay. It takes me however long my latency is.
As for loss of dps, yes, on a boss you may be at 1000dps and on the add you'd only be at 700dps. But killing an add 20% faster gets more dps from dps classes on the boss. For the 5 seconds taken to kill an add on Curator, the shadow priest loses 1500 damage, but the rogue, mage, enhancement shaman, etc get an extra second of dps on Curator, which I'm sure is far more than 1500 total damage.
Now when mana is a limiting factor, the shadow priest should stay on the boss only, unless aggro is an issue.
As for loss of dps, yes, on a boss you may be at 1000dps and on the add you'd only be at 700dps. But killing an add 20% faster gets more dps from dps classes on the boss. For the 5 seconds taken to kill an add on Curator, the shadow priest loses 1500 damage, but the rogue, mage, enhancement shaman, etc get an extra second of dps on Curator, which I'm sure is far more than 1500 total damage.
Now when mana is a limiting factor, the shadow priest should stay on the boss only, unless aggro is an issue.
Math doesn't work that way.
If you have a shadow priest doing 1000 dps to Curator and 5 generic dps classes doing 500 dps to the adds, killing them in exactly 10 seconds, you have 1000 dps on Curator outside of evocations. If the shadow priest switched to killing adds with no dps loss, your dps on adds would go from 2500 to 3500, meaning the adds now die in 7.14 seconds. 3500 dps on Curator for 28.6% of the non-evocation fight is an average of 1000 dps.
If the shadow priest is 500 dps and non-priests are 1000 dps, you have an average of 3000 dps on Curator with the priest not killing adds (5 seconds to kill adds, so 25000 damage from non-priests and 5000 damage from the priest). With the priest on adds, they die in 4.545 seconds, giving you 3000 dps on Curator.
The only reason to have more people killing adds than you need to keep up with the spawns is to reduce the time they're dealing damage. If you completly ignore the cost of switching targets and side-effects of adds such as people taking damage, an add spawning every 10 seconds is the same as Curator getting healed for an amount equal to the add's health every 10 seconds. Because Shadow Priests have the highest target switch penalty, they should be the first ones to be switched off of add killing when possible.
To be honest with you to change targets with a hunter with the autoshot changes is as easy as selecting it, with target transition you are talking about the easiest class to do it vs. one of the most difficult, there is obviously a lot of clouded vision there in how caster DPS works.
Just mention how they work to the hunter and he should be okay with it, and as said as long as you can get the adds down in ~10s you should be okay.
Well, it's easy to autoshot the flares yes, it's somewhat frustrating doing anything else though, since you don't usually get time to weave steady shots between the autoshots on such short lived targets.
That being said, I don't know why the OP's hunter cares what the shadow priest is doing, unless the hunter is being asked to solo adds and can't (which might be the case on Kargath, but I doubt it's the case on Curator). If God sees fit to grant me a shadow priest, I'd say let him do whatever he wants to, so long as he generates mana from doing it.
We kill him the standard way, not allowing him to get to 250% save once toward the end which we shieldwall through to get some extra dps.
Our shadow priests dps all the adds, but never start on the first one. We have the whole raid's dps except the shadow priests start on the first add, and the shadow priests start on the second. When the first add dies, the raid assists on the shadow priests' mob, then everyone but the mages go back to hydross. This results in very nice damage output as the warlocks get the boost from shadow weaving going into the second add and minimizes target switching from the shadow priests. I think they may drop a spell on the first add for misery, but I'm not sure about that.
"There is much pleasure to be gained from useless knowledge." - Bertrand Russell
To be honest with you to change targets with a hunter with the autoshot changes is as easy as selecting it, with target transition you are talking about the easiest class to do it vs. one of the most difficult, there is obviously a lot of clouded vision there in how caster DPS works.
With the Hunter's Mark changes in 2.1.0 a hunter will lose some DPS by switching targets as well (unless there are multiple hunters to maintain multiple HM).