"Is reverse osmosis water with added sea salt an acceptable substitute for natural spring water and Berkey filtered water?"
The quality of the finished water will partly depend upon the quality of the input water. Untainted water from a well would be perfect. Reverse osmosis works well overall, but it will not catch fluoride. It is really tough to stop, and the Berkey filters do it with a special aluminum compound. Fluoride and aluminum naturally bind together, and that is why aluminum ore plants are known for giving off super-toxic fluorine gas during aluminum refinement. The Berkey solution is a bit amusing, because they found a way to use one toxic substance to fight against another one, without either being able to escape in the end.
Hang on for some real irony on this last point. One of the things keeping the average American from falling dead by age 30 from either aluminum-induced central nervous system damage or fluoride-induced cancers is the fact that he is getting so much of both that each gets somewhat neutralized by the other.