It seems like the tape treatment decreases friction enough so the higher frequencies are more defined and perhaps a touch more extended. It really depends on the tape. The super old tapes I've treated aren't really helped as much as the newer ('70-80's) ones. I just don't think that there's a whole lot going on above 15kHz on them in the first place. I still treat them because of the potential increase in life. The treatment makes it easier to keep them free from drop outs whether it is some anti static properties it has or the slicker surface I don't know. I just know that I've never had to clean a tape again once I've treated it.
It won't make a bad sounding tape sound good, but it seems to make a good sounding tape sounder gooder.