I think the C13 is intended to open up an entirely new market for Brooks. The C15/C17 are still quite heavy compared to modern plastic or carbon saddles and I don't think a lot of people with ultra-light carbon bikes are willing to buy them. The C13 will be competitive in weight with most modern saddles and be more attractive. The C17 on my Ciocc just disappears under me, and it still has a traditional vibe (it's a Brooks for crying out loud) to it. But I'm not that concerned about the weight on a bike that would be very difficult and expensive to get below 20 lbs.