Hi, I am having a similar issue. I have a 2003 473 with Volvo Penta TMD22. The maxprop was set at 22 degrees and I could not get either top speed or anywhere close to WOT. And this engine revs high for a diesel. I have a 2.74 reduction ratio. I changed it to 18 degrees and the overall RPM has gotten to the WOT. But the speed is not there. I am lucky get get 6 knots at full throttle. I will contact maxprop again but I am thinking of changing it to 20 degrees. What is your reduction ratio and prop size and what did you end up using?
It is frustrating as the boat is way slower than it should be and I need to do a passage and you never know when you will need that extra 2 knots.
(SV Pandamonium - currently in Curacao)