Depends on what buffs are available, what the fight mechanics are, and how good a person is at executing the priorities. But yes a 360 ilvl spriest can do more than 20k dps.
with not even 340, my shadowpriest sims for 16K raidbuffed. so i think 360 should be going 20K. in fact, when i sim the 359 BiS list from simcraft, i reach 25K DPS.
26k - 29k i would guess, i haven't played much since the first week of 4.2 but at that time the spriest i was raiding with was doing around 24 - 25k in easy fights, so i would assume as long as spriest are a little op like they were then the numbers should be a litter higher than at that time.... just a guess tho.