My understanding is that a heavy weight wants a stiffer mast than a light-
but it is best to stick with manufacturer's recommendations.
Performance at the low end may be better with a softer mast, but that
can vary with the cut of the sail.
It is particularly important to have the right mast for some of the
newer high tech sails. In the last few years the mast stiffness has
generally been decreasing somewhat. For Niel Prydes the new 7.5 sails use
a mast several IMCS numbers lower than those of two years ago.
I got a 1994 Street Racer and was disappointed in its performance at first.
I had hoped to be able to use an old mast and it just made the sail too flat
Getting a softer mast made all the difference.
My experience has been that using their (Niel Prydes) "ideal mast"
a much better handling and performing sail than the "compatible" choice
particulary, with the larger sizes.
Deparment of Chemistry