What would be the drawbacks of a genetic code with 6 nucleotides instead of 4, but each amino acid could be coded with 2 base pairs instead of 3 (so the genome could be 33% shorter)?

Say, in the context of finding microbial life on Mars (i.e., organisms that evolved from the start with six nucleotides, not just taking current terrestrial organisms and swapping out the nucleic acids and ribosomes).