The 2 pieces of the puzzle that you need to supply are:
1) Where does your boat make its peak hp? (I would expect a NA 6.0L to peak in the 5400-5600 range... not sure if/how the SC affects that).
2) A known data point (WOT RPM vs. GPS speed with a known prop).
Then you prop it to turn at or near the peak hp level at WOT. That will maximize top end. If youre currently overpropped (which may very well be the case) then if you go to a prop with less pitch, you should gain holeshot, midrange power and top end as well.