There are zero performance boosts with sins on SLI mode. In fact with my SLI setup the galaxy view scrolls waaaay to quickly with SLI on and also I can't force vsync with SLI on and i get horrible tearing. With SLI disabled for sins i can actually run vsync.
Even at peark moments the video card temps barely go up in big battles. This game isn't very demanding from a video perspective.