Good point(s), Blue. Age, compression loss, take their toll on an engine. The 155 hp Johnson I had would push the ol' Pipe Dream along at 38 mph back in the day. But in later years that went down to 32-33 mph, all other things being pretty equal.
Skool, I don't much care for that extension, either. And that plate goes away down the transom, often not a good sign. However, it isn't uncommon to see yards use a plate when re-powering, just to be on the safe side, even if the transom checks out.