I'm also a 2100SD owner. After destroying my prop, I went to Mercury's prop selector and figured out what I wanted to replace it with. The "original" was very aggressive 3-balde which was good for top end speed, but the selector turned me on to a 4-blade for better pulling (tubes, skier, wakeboard) and quicker hole shot. Personally, I'd sacrifice speed for torque and stability on a sport deck.
Look at their calculator and figure out your boat + accessories weight. My 5.0 MPI is around 4600-4800 rpm WOT, but this will change depending on which prop you have on, as the knowledgeable Moderator described above.
Enjoying all 2 months of Summer in Minnesota