After getting my DK to 80 and essentially getting locked out of any available upgrades due to connection issues in 25 man raids, I decided to stop being such a sissy and level my old main character - a Troll priest.
Now...it's been a long time since I played my priest as shadow, mainly due to only having healing gear. However, since the Spellpower changes, my suit has actually become really solid for leveling. My spell hit and crit stink, but my buffed spellpower is at 1100 at lvl 70 with a truckload of spirit and mana regen - not too shabby.
When I'm leveling, I can't stand grinding single mobs, and I like to constantly be active, so this is what I do. I stand out at max range from a mob, cast VT, VE, MB, SWP, strafing away while casting the instants towards the next mob. After this sequence, I do the same thing to another mob, (casting VE whenever it's up, but ideally second in the rotation). By the time I start this a third time, the first mob has dropped, and I've only got one on me. Since I'm doing minimal casting, I can keep moving without getting too much knockback. Additionally, I don't need to waste mana bubbling or healing because I'm not taking any damage! If for some reason a mob isn't dead after SWP wears off, SWD usually does the trick.
My mana almost never runs down using this strategy, but if you get low, you can either pop your shadowfiend and keep on truckin', or cast dispersion, which on top of spirit tap gets you back to full.
I went from 70-72 using this strategy, always grinding from one quest to the next and killing every mob in my path. It took me about 6 hours for two levels.
Just thought I would offer this up as an alternate strategy for someone who finds leveling a priest difficult or tedious. Good Luck.