I've been thinking about this all morning and I think I'm going to make one suggestion. I'm not an expert on these stances by any means but looking at the sideways profile of your "Shiko Dachi" raises some questions.

In "A" I've depicted the stance as I see it in your illustration. From what I can tell, it looks like the center of gavity is well behind the feet, and that would be impossible to hold. I'm thinking you need to either shift the trunk forward so it's centered over the feet ("B") or lean the body forward and maybe still shift it a little so it's directly over the feet ("C").

I hope I'm not out of line in suggesting this, but I mayself am recently becoming more and more aware of the center of a figure's gravity as I pose and this stood out. If there's something about the stance (or the influence of the karateka's "ki") that I'm missing, please excuse me.

Before they made me they broke the mold!
http://home.roadrunner.com/~kflach/