I don't know the exact numbers, but the max damage of lvl 5 RB roll is around 300 (give or take). It is definitely worth maxing and is Lime's most damaging non-trans skill along with RB Jellies. When you first use it, you basically have an AoE around you that hits mobs, but does not push. Once Lime does the headbutt, the slime pushes exactly like how Shiny Shung pushes, so mobs will roll right along with it for many hits <3 Btw, if a mob is close enough when Lime does the headbutt, they can get HKD'd I'm pretty sure.
Freezing Lime shung, maxing it is something I've thought about. I plan on leaving it at 7 in my new build for the max 0.9 sec freeze time. maxing it would only give 15 extra damage an no extra freeze time past that. Pang lime shung, I'd leave at 1. Damage increase isn't too high and its damage is not increased in Myth. Epic in normal stage though
Try getting attack Lime Shung or another one for the invincibility frames. They come in handle in lots of stages to dodge attacks.
About potion, yes it can heal people on the ground but someone has to walk over it to activate it, like Eir's beads, to save someone.
Personally, I went for HP recovery instead of increase. That helps a bit in normal stages. I'm biased, though, because I never get HP increase on my chars. I just don't feel it gives enough.
You could try maxing RB kungkungi. Believe it or not, its excellent damage in Myth raids when everything is up in the air. It has impressive height, and it hits downed mobs so the party can keep comboing. Maxing it will give pretty impressive damage with the hax Myth multiplier.
RB shung is nice to have, but I still find the damage too low and the cd too high. I suggest leaving it at 1 if you can find another skill to max. One fatal flaw is that it tends to home on the little rocks in Myth and not hit the mobs much at all, so keep that in mind.
Sorry for long post, but had to put all my thoughts :x Hope it helps.