Your portrait studies are very solid in how you understand 3D and color, but you seem to consistently neglect proportions. Like the black guy in post 977, for instance. Maybe something to watch for, if you care?
What does doing master studies does for the artist that studying from life or photography can't do? I'd like to hear an explanation of why it's good, not a vague "I've improved, therefore it was the master studies" which is not how scientific evidence works.