Its nt that much to get correct mast flex. (Some NP use as much as 40cm)
Think you will be ok with the skinnies to get correct mast lenghth but on SD I`d stick to using the extension.
Nothing ruins a sails feel more than an over stiff mast; they recommend the combo for a reason.ie sail was designed around 400 mast and its flex pattern.
Suck it and see ???