As for Mussina's run differential vs. the rest of the league, it's interesting but only moderately useful, because the higher the league ERA, the easier to post a high differential vs. the rest of the league. And the league ERAs were significantly higher in Mussina's era than in Palmer's.
All of which isn't to suggest that Palmer isn't right about Mussina, generally. He's won 269 games, he's got a brilliant winning percentage, and I'm not sure what else a voter would need to know.