The only question is... would Lelouch be as much of a Xanatos Chessmaster with C.C.'s power instead of the Geass of absolute obedience...?
Hard to say. The Geass definitely gave him an advantage in most situations, although he has shown that he is scarily prepared for many things, like when he made a prerecording of an entire conversation in order to trick that Geass user who could read minds. THAT came down to knowing just what his opponent would say at the right moment, thus showing Lelouch can figure out most people right from the get go. Only those with personalities that aren't so two-dimensional can get under his skin... which is why C.C. tends to get to him a lot of the time.
But if he had a different ability with his smarts I think he would be able to use it effectively... if not much differently than his Geass.
