--io.file_first as a command-line parameter should work. Or, if you'd prefer, you can add the following to your preferences .cfg file:
[io]
file_first = 1

However, it doesn't matter. If you can get the unit into the game by editing the files, I'm quite sure that means you got it right. Any files found unpacked will be used instead, so there's something wrong with your editing.
That is, of course, unless you're using a separate mod folder (I am), in which case the problem might be related to inherent limitations. I haven't really read up on that though, I only know there are some problems with it.