Interesting, I got loads of diacetyl from S-04 , so much so that I only used it a couple times and never went back. Perhaps it was my method of using it. WLP002, when restrained, has a nice subtle touch of diacetyl that I find pleasant. I do agree that if you are tasting a butterscotch bomb that is over the top that this would be undesirable. I also think that rousing the yeast will help reduce the diacetyl and help the beer attentuate better. The Burton Union system was designed to keep yeast in suspension to help with attenuation and, I suspect, to help reduce diacetyl as well.