The wide platform of a cat allows for an open "slot" between the genneher and the mainsail. This accelerates the wind over the back of the mainsail and creates more lift. A mono simply is not wide enough to properly lead a headsail in order to achieve this effect.

Some cats even have articulating spin poles that optimise this effect.

Also, I have a masthead spin for my stiletto 23. It is total overkill and does not work s well as a flat modern asym. The VX40 guys have it nailed I think.