Unrolling the loop and using Anonymonk's lookup trick extracts another 5%:

int l[] = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
int c_luhn2( char *s ) {
    int total = 0;

    total += l[ s[ 0] - '0' ];
    total +=    s[ 1] - '0';
    total += l[ s[ 2] - '0' ];
    total +=    s[ 3] - '0';
    total += l[ s[ 4] - '0' ];
    total +=    s[ 5] - '0';
    total += l[ s[ 6] - '0' ];
    total +=    s[ 7] - '0';
    total += l[ s[ 8] - '0' ];
    total +=    s[ 9] - '0';
    total += l[ s[10] - '0' ];
    total +=    s[11] - '0';
    total += l[ s[12] - '0' ];
    total +=    s[13] - '0';
    total += l[ s[14] - '0' ];

    total *= 9;
    return total % 10;
}
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I doubt there is much fat left in there :)