The video I provided earlier includes a final cut-scene which clearly indicates that he have fought and defeated the Emperor. Here is another video that has no gameplay: http://www.youtube.com/watch?v=Y3T2Z-1spT8
I am not going to discuss Force Unsealed any more because it just brings back the bad memories.
Now speaking generally. Cut-scenes or not, if the game forces you to fight someone then the details of the fight are player-specific and varies (not canon) BUT the fact that there is a fight makes it a part of the game storyline. And the story of the games have the same canon weight as the novels.
if i'm getting this right, story in the game is canon, but the actual mechanics and abilities are not.
so, every sith having force lightning is not. But then there are cut scenes where the inquistor does use force lightning, as well as a many of the sithyou meet, same with the game trailers, so i guess lightning was quite a common practice amongst sith