Hi. I used to get 36-37 with a 17 pitch K series and 39-40 with the 19 pitch prop at WOT. The bottom is still mainly anti fouled+ slightly rough, so suppose will improve if/when get the energy to sort that out. Can't remember exactly but the rpms close to 5500.
Even if You don't have loose water in the hull, as the SR contains plenty of wood, there is a possibility that the plywood is wet, adding weight. Lets hope that is not the case anyway.
Check the planning surface of the boat, a rough and dirty surface will dramatically take speed away. Also weight distribution and engine trim, height on transom has its own impact on the speed.
If the prop is OK and You still get the 5500 at that speed, i think Your engine is fine and its something else.
fun on a boat is inversely proportional to size...sort of anyway