Kwango,
We used to have a 640 and 115hp Mariner in the club until last year, the the max he had from it was about 29 knots I think.
The WOT throttle shoudl be 6000 RPM I would have thought, but most set them up to 5500 ROM, as you will be aware each 1" in prop reducetion should give a 100 RPM increase, but as your figures suggest your prop would be tiny by the time you reached this!
I can olny suggest speaking to Quicksilver and Mariner (dealer).
Finally, Mark (former 640 owner) had problems with the throttle cables sticking and this meant he couldnt reach full throttle - just another idea.
Finally - if you are getting 29 knots at 5000 RPM, I wouldnt be too dissapointed, the engine wont be running flat out like this at least. My Mariner (60hp 4 stroke on a Warrior 165) runs WOT at 5500 RPM which I can live with as the engine is not being hammered then.
Adam