For me it worked well with 4.9.37 re: auto-spam. But I installed 4.9.39 before I did any more. I then concentrated upon the virii sizes as you were. I found it wasn't as successful. I don't know whether one can conclude that it works well with auto-spam such as mp3 & ipod spam, but not so well with virii.

* Some notes about how to use the spam filter to its greatest efficiency would be beneficial.