Using onlay moulds

Have seen some gorgeous cakes using silicon onlay moulds and would appreciate feedback on using these before investing in some. I know they are 2 pi r long so wrap evenly on even sized cakes eg 4 times to wrap an 8 inch cake . However my finished cakes are A little bigger because I ganache to my cake board width then the fondant goes over that. So do I need to cut down my boards and cakes in order to use these? That’s a bother because in Australia we mostly use thin Masonite foil covered boards – very time consuming! what tips and issues have others had?