They wouldn't have an inherently higher survivorship, but it would be toxic to predators, wiping them out (either directly by toxicity, or indirectly by starvation), at the same time as growing and spreading further. Natural evolution itself is unlikely to develop an organism capable of metabolizing mirror sugars, fats, proteins.

So this means people have to take an active role in eradicating this life form, if it ever comes to be.