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It may be a mistake to interpret 'treat equal cases equally' as a principle that established a default right to equal treatment unless we have good information about relevant differences. Such a default right may or may not be justified, but I'm inclined to agree with Fred that it would be a separate principle.

Most moral philosophers think of 'treat equal cases equally' as what they call a 'formal' principle or 'formal' requirement of justice. They call it that because even if we have all the relevant information, 'treat equal cases equally' by itself doesn't tell us what to do.

The principle may tell us that we should treat two murderers the same if they are the same in all relevant respects, but it doesn't tell us how to treat them. I think we all agree on that; and in that sense it is a formal rather than a substantive principle.