I know I am missing the point of the OP ,but....
...since a specific mineral load is desired for brewing various beers , if one wanted to be sure of the safety of their water then I would suggest a UV light unit that kills off nasties (or attempts to) but leaves the minerals in . What effect brief but intense UV exposure DOES to minerals I do not know . However , relatively cheap units can be had from Koi pond suppliers - since the fish are so damn valuable the water safety is often of a higher degree than that wasted on humans . :wha:
As I say ,missing the OPs point , but a valid follow on . I shouldnt think an RO and filter unit would be a lot of help unless you wanted to REALLY micro engineer your liquor from a zero start point . In which case it could be great !
SHOCKS' COROLLARY - much of what I say is a bit tongue in cheek so please dont be offended by my being wrong alot :nono: