A semi-facetious question, perhaps? Healing at least is a return to a baseline, "generally improving" is more subjective. I would tend to expect that changing any system as complex as the human brain would have the potential for improving some things, while impairing others. Not that it would impossible to find a compound that only produced changes we considered positive, without any we considered negative - just unlikely.

Not facetious at all.

If I take that molecule and find that I can acquire perfect pitch as an adult or easily learn a new language, was my brain "damaged" prior to taking the drug?

Different, clearly, but the 'damage' (or 'healing') would be subjective, wouldn't it? Since most of our organ systems return to the mean, I still suspect it's more likely that taking it when you have no reason to would be more likely to cause negative changes, rather than positive ones - but brave people are apparently willing to make that gamble.