I don’t think this quite answers the curiosity of whether starting treatment e.g. at birth would virtually eliminate morbidity, or whether it only slows the decline once it has started.

Consider that the disease typically manifests in your 30s — does this mean it would begin 4x later (and thus basically never manifest), or that your 15 year progressive decline from ~35-50 would take 4x longer (giving you a normal lifespan, albeit perhaps with some limitations in your later years)?