...with our 40 hp Yanmar a number of years ago. I kept looking at the exhaust elbow (which was ultimately the cause of the problem!) and dismissing it. I popped the injectors, checked compression (I knew it wasn't a compression issue but covering bases...) and just drove myself crazy until a parts guy who listened to the symptoms said "you are missing the clog in the elbow". I went back to the boat and started the engine and I knew he was correct!
Pulled the elbow off (takes 5 minutes on our boat) and looked at it from the back side (from the water muffler side) and couldn't believe what I saw!!!
The exhaust path was reduced to about the size of a dime! After I saw it I realized that it was actually sounding restricted but I had not noticed as it had changed slowly over time.
A new elbow was installed and the world was right again!
That was our experience anyway...