I agree that at some point performance suffers. It's sort of a personality thing too. Some people should use more castor because they need every blessed last rpm they can get. You know, the needle tweakers! When I ran too much castor in my 2 stroke YS pattern engines it just about ruined them. They ran like crap, and carboned up real bad. When an engine developes carbon or lacquer build up it gets ugly too. The composition of lacquer is such that when the engine gets hot, the laquer gets gooey and wreaks havoc with engine performance. When the engine cools all seems fine and on goes the vicious cycle. At that point the engine needs to be torn down and massaged a bit. When I ran too much oil in my Rossi and MVVS engines they did not perform well either. My style of flying dictates total oil in the range of 17%, one fifth of that being castor. Walter Morris told me that years ago to get my profile engines running the way I wanted them to and he was right. I miss him and the old Morris Hobbies.
Whew, did any of that make sense