The scorpion poison didn't cause any changes to her Mutated Beast once the Klock grew its fifth head. But, it still mildly strengthened her body.
A few days later, Xuna realised that this poison and the leaves worked like a drug that allowed her to skip months of training in a day, thanks to them working synergically with her Nacre.
This allowed her to truly tap into the physique of a Horned Human, capable of jumping to a height of two metres easily. And this was just the growth of a few days.
Even though there was no body tempering effect in their power System, that was only true until Stage 4. Once they reached Stage 5, they could transform into their Mutated Beast, able to rely on its tough body to tank attacks better.
So, by reaching the peak of her physique now, Xuna would be able to tap into her physical strength at Stage 5 better.