dont think the issue is as much as the healing but rather how much gear has progressed now over our level. in the level 50 hm everyone started with 49/50 mods. in the level 55 hm players minimally go in with level 61 to 66.
i would like to see if the 55 hm would be doable in 55 columni which would put them on the same level as the 50 ones starting level gear wise. but that gear isnt no longer available.
the argument could be made that the 55 hms are made with this gear discrepancy in mind. but then just how accurate is this if so.
It is doable as a healer in full Columni. Additionally we didn't start with Columni or even Tionese when I first went into 50HMs in 1.0. We had questing gear and some normal 50 FP gear.