Hi Paddy,
The North should be fine with the Storm. Actually IMCS isn't the full story, there used to be things like "Flex tip", "Constant curve" which correspond to where the mast bends most, and have nothing to do with stiffness.
Everything you ever wanted to know about masts but were afraid to ask is here:
www.peterman.dk/start-windsurfing-gb01.htm