The Mercury web site does not list a hub kit for VP for the Spitfire prop. The only stern drive they list is Merc. The rest are for outboards.
Get a nice Solas 4 blade (if you want a 4 blade ) and have no problems with fitment. It will perform as well(or out perform) a Spitefire prop. VP makes 4 blade props too.
Frankly, I think that you'd be just as well off putting a nice 19 pitch 3 blade on there to gain your rpm back with a boat load of people and gain as much "hole shot" as you would using a 4 blade, without losing top speed(with the heavy load).
Right now you're propped for a light load, and you have to keep in mind that one prop can"t do it all. You prop for a specific situation, and when you change the situation(load), it always requires a change in prop pitch.
That's why most of us end up with two props; one for light load running around, and the other for heavy load duty.
Your 21 pitch does fine for light duty, and a 19 pitch is needed for those days when you've got the gang along and/or need pulling power for water sports.