The WOT for that engine is 3600 RPM. Yanmar says you can run it at 3300. I would personally not exceed 3200 RPM. At 85% WOT you are at 3060 RPM, which would be a good top speed. At the 2500 RPM you are still at the peak of the torque curve, and are putting out about 10 hp at the prop. The fuel consumption would be about half a gallon an hour. At 3200 you are putting out about 20 hp at the prop, and 1.25 gallons an hour fuel consumption.
Since the engine is Natural aspiration, you don't have to worry about carbon in the tubo (and some other problems). As long as you keep the temp up to operating specs (and not over) the engine should be happy and last a very long time. I would personally favor the 2500 rpm where you say the engine sounds good....
