I am rigging a 4.7 Ezzy on a 430 skinny Ezzy so a bit of mast pokes through the top. I guess a 400 would be better but what difference does it make?
Will my setup be stiffer, more or less powerful, twitchier or don't worry about the difference?

If you are a big guy 90kg plus you might be ok with the stiffer mast.

Typically when you rig a sail on a mast that is too stiff it rigs very flat. This is because a large majority of wave sails use luff curve, to an extent, to create their depth. When a mast is too stiff it doesn't bend as much, hence the luff curve is less and they rig a lot flatter.

Ezzy's are a little different because they don't generate their shape purely from luff curve. This is why they rig quite differently to a lot of other sails. As NightsTalker said this makes them a little more tolerant to incorrect mast stiffness. It still won't be ideal and the mast won't flex off like a softer mast as easily but it still should be 'useable'. But as you say Cruiser it will be stiffer, twitchier and probably lose a bit of top end.

thanks for that