Sooty transom
Your dealer is may be right, but overloading the engine does not relate only to RPM. If the boat bottom is extremely fouled, more HP is required and the overload can come on sooner. If the engine is not running properly, it does not develop full HP and thinks it is overloaded sooner. The problem could relate to the injectors. If one was dripping (not shutting off between squirts) excess fuel would be present the next time that cylinder was supposed to fire and it would not all be burned. If the injectors were not calibrated properly, one or more could be wanting the engine to turn faster than the other(s), hence exhaust with partially burned fuel. The Universal M25XP has more than enough HP for the Catalina 320 if all is in order. 2500 RPM sounds like a good cruising speed. If you have too much propeller you could be approaching over load sooner. I like to plot a graph of RPM vs hull speed, on a calm day. As RPM increases, so should boat speed, to a point. The graph usually makes a near straight line until the point where it requires extra RPM for a given increase in hull speed, ie: the line bends. The start of the bend is the most efficient speed to turn the engine. That speed should relate very closely to the theoretical hull speed. In the mean time, keep on scrubbing. That black stuff can do bad things to gel coat if left on too long.