Well with the 25% buff and tricks it would only require 10000 spread-sheeted dps, so it's not really that unbelievable considering he has mostly 264's. I have some 277 pieces in my mutilate set and I pull 16441 dps on festergut(http://www.worldoflogs.com/reports/r...?s=2479&e=2674).
It doesn't seem unreasonable to do 13k in his gear with that perspective.