It's possible that an Asha'man could heal Siuan and Leane back to their full potential without the need for severing them again. The reason I say this is that when Rand receives his second never-healing wound from Fain he is initially healed by an Aes Sedai (Samitsu, though I may be wrong) but would almost certainly have died if he hadn't ALSO been healed by Flinn. I took this to mean that the way they do things is sufficiently different that one doesn't prevent the other from having any effect.

I don't see any plot-driven reason why this should happen in the remaining books though, as Siuan and Leane have adjusted to their new level.