Might I suggest an unusual alternative for gear/gem choices? Early on if the priest isn't hit capped, it might be in their best interests to try a bit of gem swapping (i.e. if they haven't gotten spellstrike, or just haven't had quite the right gear luck quite yet) that they try using vitality to boots, and using "Swift Starfire Diamond" to satisfy the run speed survival issue, and also since next patch MSD is getting nerfed into the ground, its necessity is slightly diminished. To satisfy the two yellow gem requirement and the hit issue, using "Veiled Noble Topaz." I don't normally recommend using alternative gems, but this might be only a couple dps lost, and survival necessities covered, dead shadowpriest don't dps (unless we still have dots ticking )

I'd argue that an equal factor in your DPS is how good your tank is at in producing threat. This is probably more important for spriests more than any other class since SWD is a fairly high threat spell, considering that you're going to double dip in threat as VE heals the damage you take from it back up.

As for casting priority, I always go:

VT
VE
SWP
MB
SWD
MF

i.e if VT isn't up, then my first priority is getting it up. My primary role in a raid is actually not to do DPS, it's mana regen via VT, and in more than just a few fights in T6 content, healing via VE. Having those 2 effects up is paramount. Of course, doing DPS is right behind that, but the mindset that VT+VE is the only reason you come to raids is a great way to force yourself to focus on making sure your uptime on those is as great as possible.

Any fight you need to do some small movement? Do it when you need to cast a SWP, or SWD or VE, since you can cast those on the run, and have a 1.5 sec global cooldown before you can cast something else anyways.

Something else to add would be considering DoT uptime. If you use WWS, this is easy to determine if you go to your character's page and see the time you were present for. Lets say for a Void Reaver kill, you were present from 18:52:06 to 18:58:32. That's 6 minutes, 26 seconds, or 386 seconds. Lets say you had 105 ticks of VT. VT ticks every 3 seconds, so 105*3=315 seconds of VT uptime. 315/386 = 82% uptime with VT. Note: You will never hit 100% uptime with dots, there's many factors involved to how your uptime goes. It may be a movement fight, in which you'll find out your SWP uptime will be better than VT, since you have to stop and cast VT and it may come at a time when you can't afford to stop. You can't exactly stop and cast VT if an orb is coming right at you. The start of the fight is usually when the MT engages, and dots aren't up right then as well, and for the last 1% you may not reapply dots. Finally there's going to be a bit of a gap inbetween your old dot and your new dot, since you defintely do not want to overwrite any of your dots until that last tick has occured. Remember that when you apply a dot, the first tick doesn't occur until 3 seconds in, and the last tick occurs as it expires. To illustrate the concept of clipping your dots and why this is really bad, consider how you should approach VT:

0 sec: You start casting VT
1.5 sec: VT lands on the mob
4.5 sec: Tick 1
7.5 sec: Tick 2
10.5 sec: Tick 3
13.5 sec: Tick 4
15.1 sec: You start casting VT
16.5 sec: Tick 5, VT falls off the mob
16.6 sec: VT lands on the mob

Now if you had started casting VT at 14.9 sec, it would have landed at 16.4 sec, costing you the final tick of your previous VT and cutting your DPS from that spell down by 20%. In short, have a dot timer, know when your dots are going to expire, and make sure they're reapplied right after they fall off so you don't clip that last tick. As for what your dot uptime should be for any particular fight, that's really hard to say. In general, anything under 80% means you definitely have room for improvement, big movement fights (Void Reaver at times) notwithstanding. SWP uptime should be higher since it can be cast on the run, and there's less casts of it needed due to the longer length. The #1 thing for an aspiring shadow priest to work on is dot uptime. So is the #2 thing and the #3 thing! So I'd make this a big priority in the guide. A dot timer is mandatory, and examining your uptime via WWS or other tools is almost as mandatory. If there's another shadow priest in your raid, definitely compare your uptime with theirs. See how much mana you returned via VT compared to what they returned. Healthy competition is a good thing! Comparing my uptime to other shadowpriests and grinding my teeth when mine wasn't quite as good on a particular week was how I got better and better each week. Any retard can mash the MB or SWD key when it comes off cooldown, or mindlessly spam flay, but to actually track your dots and efficiently reapply them is what separates the middling shadowpriests from the good ones.

Really....so mob +3 drops to 83%? Really gotta wonder what people are thinking when they design these mechanics.

I'd rather have the shaman personally... WoA is still better and you get other benefits such as mana spring and either mana tide or totem of wrath. The only fight it's ever an issue for me is Council, simply due to being on opposite sides of the room.