Marcel...

That's not what I'd call my "standard" hand position, although for certain open chord shapes, I do seem to like my thump around the neck. Generally, though, it's more behind, especialy with barre chords.

As to single-note stuff, I'm just beginning this, and I do keep my thumb behind, The dexterity issues that I speak of, to be more clear, have to do with achieving the LATERAL spread or SPAN of my fingers along the neck... while maintaining good thumb position. This is as yet difficult for me, and may have to do with a tiny loss of muscle strength in my pinky (the muscle that moves it away from my other fingers), and the lateral side of my ring finger, that makes it harder, and I'll have to overcome that little challenge. But I'm home today a little under the weather, and I'll practice these more. :) I am convinced that this is a minimal inconvenience and can be overcome with continued training of those muscles.

TC