Japan and NGDP targeting: A victory for whom?

Well, if it happens the Japanese obviously.

But would you consider it a victory for Krugman or Sumner, and why?

One way to call it would be as a "policy victory" for Krugman, a "target victory" for Sumner, and a "theory victory" for both New Keynesians and monetarists (less so for Old Keynesians but they're either dead or harmless blackboard stick figures at this point).