Ok, been using my 70 warlock to lvl my priest/hunter duo and 2 friends. We're up to Scarlet Monastery now that they're 20+.

At around lvl 23-24 for my alts, and 29-30 for my 2 friends I was pulling in up to 40k XP an hour.

The warlock has around 1200 shadow damage + trinkets + any pots/food buffs. I just run around and throw Curse of Agony and Siphon Life on everything, don't forget to leave fire shield from the imp on auto-cast, helps alot more than you'd think... this works out to be the most mana/speed efficient way to kill and stay at 100% life/mana with a lifetap giving back around 2k mana, or dark pact doing the same.

Fastest clear time is under 12mins for the Cathedral, 8mins for the armory, I only do the library if someone needs the key or loot. It only gets slower when one of my friends on follow manages to come too far into a room and aggro half of it, and I need to run around picking it all up ... I kill the last bosses and loot the 2 blues to sell and cover repairs, the others loot the rest, then trade me some stacks of cloth for gold etc...

Prior to SM I ran Deadmines from 10-15 or so, 15-20 in Shadowfang Keep, 20-30 will be SM, then 30+ in uldaman. I'll update what I do at 45+, but I have some ideas