After a while shooting K400 I searched if I could use it at ISO 800. I founded out that even at ISO 800 the result was very good.
Then I looked if I could shoot it at ISO 1600. I actually found photos in the internet from photographers who have used it at ISO 1600, but I could not find the development time with Ilford HC developer which I like to use. So I had to try by myself.
I have being enjoying it at 1600 which I think is the perfect ISO for this time of the year where I live. It is impressive that it has little difference of quality from it original ISO 400. Even the grain I didn't notice much difference when I expecting get a more grainy result. I have being doing a more aggressive agitation during development to get more accentuated grains from K400, which I thing is perfect for my street photography language.
I want to try it at ISO 3200 later on, just to compare and see where is the limit.