As I tried to point out above, headstay sag is not the important thing. Sail shape is the important thing. That and not breaking the boat. If the sail cannot be flattened with the maximum backstay load the sail need to be recut or replaced.
And...masthead masts can bend to flatten the main just fine. This one will easily go to several diameters. Just depends on the design, and not too heavy of a spar. The typical heavy cruiser mast
will not bend much, though. Maybe half a diameter before head
door cannot be opened.