The one paper I was able to find in regards to feeding a cooked meal only studied the cost of digestion, nothing about what kind of nutritional implications there were. If you’ve got a snake taking f/t, there’s really no reason to mess with that.
That said, sous vide is not the method you should ever use for cooking a whole animal unbutchered, it’s a recipe for disaster because you’ve got stomach contents and feces. Sous vide cooks gently, yes, but that means it takes longer for the ideal internal temperature to be reached and all that time the bacteria remaining in the rat’s digestive tract is essentially in an incubator, causing putrefaction. You’re just going to end up with a bacteria bomb that is likely to make your snake very ill.