The singular shadow priest is just the old one. Very inefficient. I think most of the people use singular to lvl up a char, once 85 they normaly use another CC.
Singular has still before andf after mind spike. That means, when u have mind spike, the only thing the CC do is casting Mind Spike. This is old stuff. If i look on my lvl 85 priest, Mind Blast do 3x more damage than Mind Spike.
Devouring Plague is a proc now, u can't use it all the time. Vampiric Embrace is now a skill to heal you and party members.
Low lvl priests have issues with mana. The singular rotation as it is now, makes the priest to sit down after killing 1 mob to drink.
For leveling :
On normal mobs the priest use only 3 spells : Mind Blast, Mind Flay till 25%, SW

(between Mind Flay, Mind Blast if of cd).
U don't waste mana to preshield for mobs that u kill in 10 seconds.
Now we can heal in shadowform without coming out of it. Make a rotation that we can cast a renew when we need health.
Only cast PW: S if health lower than 50% followed by a renew.
If we have elite mob or boss, then we use Vampiric Touch, SW: P in the rotation.
We are priests, so in the need rest section, remove eating for health, instead us a heal spell.
Use wand as option ? No sense at all since 5.0.4
A shadow priest is a killing machine, with the current singular the only thing a priest do is kill 1 mob, sit down to regain mana, kill 1 mob, sit down to regain mana .....