I think the best racing masts, especially for large race sails are made by Maui Sails. They make their SRS 100 masts in sizes frm 430 to 550. They are a little heavier, but they don't break as often as the other brands.
Of course this also means that they are made for the TR-4 race sails which range in size from 4.3 to 12.0. These sails are as good or better than any race sails on the market. However, I think the masts have the same design characteristics as the Gaastra masts, so they should work with both the Maui Sails and Gaastra sails.
I am just an amature sailor / racer that uses the Maui Sails products and think a lot of the company and products.