Hi James,
It will work. You don't tend to notice the mast flex unless you have ridden it with a stiffer mast first. You might have some difficulties during gybes etc. but for starting it will be ok.
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Gunnar
hi Gunnar . . . is the difficulty with gybes related to the 91cm mast being not stiff enough or just that James said that he was still working on gybes?
I am in same boat as James . . . though I can gybe and tack on a foil.
I already have the 679, 633 and 720 but only the 91cm mast and was hoping to get away with just adding a bigger wing like the W1100 for winging without the added expense of getting the 82cm mast.