Im preety sure he was. otherwise how did he manage to turn Revan and defeat a group of the most powerfull Jedi and turn them too. Even without training his streangth and power without sith training managed to srip his own real father from his rank and power at young age too. and that after gathering followers. Who have been described as more powerfull and who would fear him if he wasent they even say he is the most powerfull in the JK story I dont remember exactly what was sayed but it defenetly sounded like that expecially when it comes too them saying "Imagen what he could becom if he destroyed the galaxy" something like that.