I have an 1800sr3 with the 3.0 engine. I changed my 21P prop to a 19P and that made my boat run like a dream and pulls well etc. I think your prop is way too small for the 1900SR3 with a 4.3. I would start by changing to a 21P three blade prop and see what difference that makes. With a 16P prop I imaging the engine is over-revving and not getting any decent grip. At WOT make sure you are getting the RPM's in the correct range for that engine.
I hope that helps - Let us know how you get on.
Another Day.....Another Bay!!