I appreciate everyone wanting me to use the library mode and I think I explained why I don't want to.
The alternative I could consider is going into each of the nfo file and reset the genre to my taste.
Now in terms of performance, I can garantee you that I don't have 500+ movies in one folder, the biggest one is probably 40 so I wouldn't worry about this. If it can read 20 songs ID tags (btw from binary files, not well structured xml files) then it should read instantly 50 nfo files.
Even if it takes a few seconds per folder, the benefits of having this option as a nice skin would be tremendous...
BTW, if I use library mode and open the Action genre, it takes more than 10s and then I have to scroll through about 100 movies. In my Action folder, I have only 38 (other could be considered action are in adventure, war, sci-fi, etc...)
I would really love to be able to contribute to the programming xbmc team but it would take me way to long to get in it compared to one of you experts for which it would take probably 30mn...
If it can be a toggle video option in Settings/Video/Files/ then it could be up to the user to choose.