This year, the results of the PPE test won't be released until after everybody is interviewed and selected, so that they can see if the high scores correlate with the people who eventually get in.
I also read Marx, Mill and Machiavelli (and Sartre and Nietzsche (yes of corse grate speling me...)) and none of them did me any harm- mostly because you can choose to read quite short books from each one! Picking up a pop economics book like Freakonomics might also be something to consider, as its very readable and gives you another thing to put on your PS and perhaps talk about at interview.
I like your confidence, but make sure it doesn't turn into overconfidence- there are people with great grades and lots to say for themselves who get rejected regardless. it's great that you're articulate, but what you say is far more important than the way you say it. Don't be afraid to stop and start and think about what you say, even if it feels like you're constantly pausing or rambling slightly or whatever; at the end of the day, they are looking at how your brain works, not how your mouth works.
That said, you certainly have as good a chance as any, so best of luck, enjoy reading your 3 Ms and I'll hopefully be seeing you at Freshers' Week next year!