If I defuse Fusion Drive, I can install HS with the new filesystem right now.
I think that a pure SSD is better at all. I think I can manage the SSD and the mechanical HD manually and correctly, to get the maximum of the set-ups without the operation of automatisms that in theory could sometimes make slowdowns on Fusion Drive, !'m wrong?
Yes and no. Technically you can't manage the data the same way the FD can. It operates on a block level (of data) while you deal with files.
For example say you use photoshop and its a 1000mb program, however you dont do anything advance in it. The FD could move 500mb (just an example) of the program you dont use onto the HDD saving you 500mb of SSD space. You can't manually split a program like that.
Another example, say you repeated watch the first half of a movie you have downloaded on MP4. It could move the that first half of the file to the SSD and the part you never watch to the HDD. Again something you can't do.
However if all your stuff fits on the SSD and you just use the HDD for media (or nothing) then unfusing it would probably be better. You wont have to deal with the initial slow down while it learns either.
Again, I'm not saying the idea of unfusing is bad, just there are benefits of a FD too. In my experience with a 2tb FD I ended up preferring to let it handle where the data ended up. While I didn't particular like the idea of a commonly listen to MP3 making its way onto the SSD if I listen to it enough it made sense to have fast access too it. I also didn't like the idea of portions of my commonly used apps ending up on the HDD because I never used them, because I felt that I might one day....I never did....