Wow, excellent job Fincks!! I agree, you should spend some time playing around with texture brushes - find one or two you like and just mess around with them on a new layer so you can always modify or delete it. Try it over one of your finished studies, so it doesn't count against your time (I stopped keeping track of time altogether - I work better when I don't have a ticking countdown and sense of intense urgency in my head). Remember it's not about how fast you do this - it's about how well you're learning to do it. As Dave Sim says, first you get good, then you get fast, then you get good and fast. You can't bypass the steps. Texture is the missing element in your work - it's amazing how everyone's work improves when they start adding texture. Stuff goes from all looking like smooth plastic to having an organic surface.
Keep up the great work!