Masts influence rig performance.
It is best to use the recommended mast for the sail.
If a softer/shorter mast is used, the rig becomes more flexible. Handling may become easier in high winds, and the rig may become easier to close down on the board (closing the gap).
A stiffer mast will have the opposite effect; the rig will become more difficult to close in the upper wind range, will go upwind more efficiently and become more light wind oriented.