after some crappy luggage crew broke my mast I was forced to use a one coming from a different set - 1 IMCS difference was HUGE! especially with closing on the overpower line.
Using a stiffer mast may cause the sail to lag with reaction (did try UltimateRace with 30 IMCS that went with my Arrows, with Severne SSR - 29 req. - wouldn't open like it should) - same goes other way around -> sail req. stiffer mast (above mentioned arrows with the severne mast when they broked UR) would twist too much plus a proper downhaul could not be done in either case - wrong shape
final sentence - totally NOT recommend doing that - even at the expense of a heavier mast (I did bought a used 30%C Lance with 30IMCS from a local - thank God he had one!) do always use the proper stiffness!