Brian, it would do 30kn no probs - I expect it could have even had more pitch on it as it hit WOT max no probs - but towing the toys etc required more pull than top end.
Is the 50 Evirude a newish one? I believe old outboards were rated at the fly wheel and new ones are measured at the prop - therefore and the difference between and old 50 and new 40 isn't 10hp! But if the revs are rising and the speed isnt you may need to trim it differently or you have a cavitation issue or the leg is the wrong length.
These boats were foam filled and the foam could get waterlogged - mine was ok but it could be worth getting the boat weighed as it could be carrying 1/2 ton of water!!
On JoJo I have a Yamaha 50 4-stroke 2003 - beautifully smooth and quiet.
Rob