Q: Why is the MFT not placed at the very beginning of the volume?
A: The position of 30% into the disk is the most accessed area on the disk, because of the zone structure. The boot files are on the very beginning, then we have the most important files for the system.
Q: Directories are placed directly after the MFT. Basically my question is - what's their role on an NTFS voulume?
A: A directory is basically a file which holds the list of files that the directory holds, with crossreferences to the MFT. You will have performance impact when you place the directorys very far away to the files that are contained in it, so the head must move more far. In praxis, the most impact will be while open a directory with a huge number of pictures in it.
Q: Boot files and System files. Are they more important regarding performance than MFT or directories respectively?
A: This answer is capsuled in the other two answers.
And the point with the zones, I currently don't really get what you mean...