I think it will be a good thing for you to have a second bike. However, I don't think changing the bike will significantly improve your performance.
In the context of a group ride, you're already drafting so the aero advantages of something like an R3 won't make much of a difference. Unless your rides are really fast or seriously hilly, it's also unlikely that the 1-2 pound frame weight difference of the newer bikes will make a difference.
If you're doing solo centuries, you might want to try aero bars; that's just about the only mechanical change you can make to improve your performance. Otherwise, things like an aero frame or aero wheels or a minor weight change are likely to reduce your century time only by a few minutes.
Another thing to consider is that any improvements due to a mechanical change to the bike is essentially a "one-shot." I.e. if somehow you manage to get a 5% improvement from switching bikes, then you've just gotten to a slightly higher plateau and you're done. In comparison, by training properly you will hopefully gain continual improvements....