You definitely have tricky but interesting questions... I don't believe 'so... as....' would work. At least it doesn't sound right to my (non-native) ear to say 'she is so good as her mother'. I believe it has to be 'she is so good' period. Or maybe 'she is so good, as is her mother' would work but it has a different meaning : she is good, her mother is good also, but
Waïti is right, you can't say "she's so good as ..." in a positive sentence. In a negative sentence, both are used, "not as good as" or "not so good as" (they express the idea that she's less ... than).
Thank you PieAnne for the clarification. I had no idea both alternatives would be acceptable in a negative sentence. I'm learning something everyday with englishforum.com... W.