Hi Neil,
A 400 mast will suit sails around 4.5 sqm, while a 430 will be usable from about 5.0 to 6.0sqm BUT you need to take in to acount the stiffness of the mast which is measured in IMCS.
If you were to rig a sail that should be on a 430 on a 400 plus a 30cm extension the mast may be too soft resulting in the luff being too floppy and the sail won't rig correctly
You could try downhauling the 6.5 harder and outhaul it more to flatten the sail off when it gets really windy
Good luck!
Steve