For a number of years now, I have been using 30-40cm of extension in my older 7.0 Windwing and my new 7.1 Hansen with great success. Although a 460cm mast was recommended for both these sails, a 430cm mast was cited as an alternate. Similarly, I used to go with a 460cm mast for my old 8.1 Windwing Race sail that recommended a 490cm mast.
Now, I probably wouldn't have normally deviated from the optimum recommended masts, but the path to the alternates was unexpectedly found due to broken masts. This led me to some experimentation with shorter masts and longer extensions. What I found is that I liked the result. The only important thing to keep in mind is adherence to the proper bend curve masts that the sail designer recommends. More and more today, going with recommended bend curve has increasingly been a prerequiste for optimum sail performance. Although it's sometimes it possible to use a different mast brand than recommended, it's often very dicey. I would hate to confront the dilemma that many NP sailors face after repeated X9 mast failures.