Is tamari and soy sauce a 1:1 substitute?
The answer is both yes, and at the same time no.
The problem is that there are a lot of types of soy sauce (see What are the differences between types of soy sauce? ), and many of them are not good substitutions for each other.
Depending on what type of soy sauce you're replacing, Tamari may be a good 1:1 substitution, or it may not be.