Thanks and welcome back
Yes, I already thought about removing the inner line - that should be the right thing to do. However, you shouldn't make the outer line thinner. In the scaled-down version the diagonal parts of the outer line are more visible than the vertical/horizontal parts - bad. Changing the color to something slightly lighter than black (maybe #404040) should be enough.
Making the cross transparent on the font - no, I don't think so. That looks wrong. Also, the nice shadow/reflection effects only look nice on the big versions that aren't going to appear in the interface. You can't recognize them as shadows in 24x24, and even less in 16x16.
The text is somewhat too big, compare with http://images.google.com/images?q=stop+sign
. I don't think it should be bigger than the vertical sides of the octagon.