I thought AoE capping worked on the premis of a max/min coefficient.. initially a single target takes full damage, and the more you add the lower it goes, but it would never go below the base damage in the description+ a small percentile from talents/gear. so for this one it should be a min of 500 per mob.. so once you reach this limit, the damage rises linearly with every mob added.

unless they are completely changing the AoE system :S