Juzz, it depends on which book or reference you believe.
If the -1C is a derivative of the -1D, they should be basically the same.
If the -1C is a derivative of the -1A (which is possible, and would explain some things), then its conceiveable that the -1C is slightly lighter and would have less drag.
I personally don't know which way to believe, because I have seen convincing evidence from both positions.
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Vermillion
**MOL**, Men of Leisure