My boat (with MD2030D) was purchased with already 3000 hours (like Symphony's boat, an Ionian ex-charter). The elbow was probably never replaced and after about a year it began to exhibit difficult starting - but I managed to figure out a certain starting "routine" and did not give it much thought. Then, about another year later, it began to lose power - simply could not rev up.
This is when I looked into the issue and found the badly clogged elbow, which was replaced to a new one.
Several years later It again got difficult to start the engine - this time I cleaned the elbow immediately and things went back to normal.
So, my experience is that the first signal to an elbow blockage is difficult starting which is accompanied by a "blob" of soot on the water near the exhaust. If there is no problem with fuel then these engines should start very easily and no soot.
I think that on my next cleaning cycle I will replace to SS - keypart.com is selling an SS elbow at about the same price as the original cast iron.