If the game didn't give you Tabs to alter stats though, Magus would be one of the strongest characters naturally since Crono's magic only reaches 46 at max level, they would be on equal terms, though Lucca's standing in the game would go up.

In regards to the significance of Magus' super versions of everyone's spells. It is important cause it allows him to be versatile and efficient. It wouldn't mean much if you are simply Flare/Luminaire/Dark Matter spamming but if you actually care not to blow through ethers (despite them being numerous) Magus allows you to blast your way through dungeons with little effort and little MP cost. Until you get some of the final spells in the game Dark Mist is probably one of the nastiest spells in the game.

In terms of storytelling and character development, I kinda wonder what the point is. We see everything in regards to Magus' past so its not like he would give us any real insight to who he was. He was a bratty little kid to begin with so its not like his personality is really a stretch from his childhood. In regards to his speech with Zeal, once again we have to remember that Janus never got along with his mother and since he was taken at a young age any attempts to foster feelings of love or hate for her are gone. His dialect of disgust is fitting cause his last real contact with her was as the Prophet where he got to see first hand her greed and how much of a tool she is. In that state of mind, would it matter if he made some great proclamation? It would be like talking to a wall.

For the rest of his dialogue, if they had written more it would most like be nothing but snarky comments and "cool" boasting so I highly doubt it would somehow flesh him out. It would only give more fodder for the haters to complain about and more annoying lines for the fanboys to misquote.