Although I am not expert to increase the speed on our boats we have done the following:
a. Lift engine up one noch as you still don't get full revs @ 500 since your max is 5500
2. If engine over revs at that hight (i.e. over 5500) increase prop pitch (in your case from 19" to 21").
3. You need to play with props too. Same 19" pitch props do not give same top speed or performance.
4. More importantly you need money to spend
Boating is an expenssive hobby and business too