..and just to be clear, it's your manifesto that needs to be 300 words or less. The rest of what you submit can be as long or short as you like. As usual, I'd take the time to write less. The judges - like clients - will appreciate brevity and clarity.
More importantly, when you edit, precis, distill etc, you tend to find all sorts of little wrinkles to iron out. It forces you to think a little harder and better.