Gersdal, this is a very, very cool lay out, and I appreciate your presentation here. I might disagree with you on small details but they are details that are irrelevant to your main point, so overall I think this is great. It should be a sticky.
I know you don't start threads too often so it's cool to see you come out of the wood work with something like this that I think will be helpful to many.
I understand the problems that can come with assigning "levels" to understanding of improvisation, as it's purely subjective.
I really hope nobody gets too invested in a discussion of what is or is not "advanced". It will get us nowhere. Instead, looking at this simple layout of understanding I might rate it as 0=guitar player that knows major, minor, and pentatonic scales and can improvise in one key and 5=gersdal.
I might simplify things a bit, maybe assigning different 'levels' using my own experience and observations of others.
A huge factor is the influence of rock playing and rock "training" that is common for guitarists, and that's plays such a huge role in the problems guitarists face when starting to try to play jazz.
So, my take:
levels of improvisation in relation to modes, common progression for a modern guitarist, focusing on the relation to modes:
0 - knows nothing
1 - knows pentatonic (to play rock)
2 - knows major and minor scales but has a very hard time improvising melodically with them (probably because of lack of aural skills as well as a lack of vocabulary that uses those scales)
3 - learns a few modes but similar problems as #2, uses modes over modal vamps and rock songs
4 - can play melodically with modes, make logical, melodic statements
5 - develops an interest in jazz. Is faced with many ii V I progressions and is mentally stuck trying to assign a mode to each chord
6 - From this point, it seems people diverge in different ways. For me, I lept off from this point by getting into bebop and learning a lot of bebop vocabulary and completely eschewing chord scales/modes. For some, they take a more educated and logical approach to CST, probably combined with transcription, and learn to navigate chord changes passably. There are other pedagogical approaches as well, but we're talking about Jazz Standards here and all approaches to probably boil down to, like you said, looking more at key centers rather than modes for individual chords.
7 - From #6, we're talking about players who can play through tunes and have it sound "fine." From here there are obviously an infinite number of directions people can go, stylistically, technically, etc, so I feel good leaving it here. Again, this is ONLY in relation to an understanding of modes. Completely leaves out issues of taste, phrasing, rhythm, voicing, style, etc
“You rarely have time for everything you want in this life, so you need to make choices. And hopefully your choices can come from a deep sense of who you are. ”
-Fred (Mr.) Rogers