Hi Turbojoys,
I think you’re off to a good start. I love the idea of master studies in ZBrush.
I’d advise you to drop a subdivision level or two and really work the major forms until they match the painting. Your going to fight lumpiness if you work it at this level, and I think some of the issues that the piece has may have come from bringing it up too high too fast, trying to get broad, structural form with what’s really supposed to the surface “scumble” clay.
The main area that’s off is the bridge of the nose and the brow. You seem to have a thin, fleshy ridge sculpted over each eye instead of the very subtle angle of bone that arches out from the bridge of hte nose. Also, if you compare with the painting, his forehead and nose bridge appear very smooth, and you’ve got a big vertical ridge running up his forehead, and a tiny cleft chin between his brows 
The same thin, fleshy ridge/flap phenomenon is going on with the naso-labial crease. The guy in the painting feels like a healthy, slightly plump man, and the man in the sculpt feels like he’s lot a lot of weight rapidly, and he’s a bit gaunt. Convex curves are rare on the human body, and you’re caving in his cheeks near his nose.
Watch how sharp the angle of the jaw is in the sculpt, and the round cross section of the sterno-mastoids in his neck.
I can’t wait to see more!