No need to "nerf it" just put a reasonable complexity on it for the effects. It's not like there is a huge penalty to cast for the complexity being higher.
And in desperate situations a bad critical failure can be as good or better than a spell going off as intended.
OTOH leaving the complexity but "nerfing it" by taking away the physical damage effect at 80+ doesn't change the usefulness of the spell at all. Just a sudden increase in spell power that makes it overpowerful for the complexity.
More reasonably if the spell is left at the same complexity then at ML80+ the effects are the same as ML80- except that for every die of ethereal damage one point of the damage is actually physical and doesn't recover as fatigue. Maybe even make it a bonus effect at ML56+ instead.
IMC I don't allow any spell bonus effects that at ML70+ or higher suddenly multiply the effect of the spell by 2 or more.
A spell at ML66+ might go from being touch to SI feet but never from touch to ML yards. Preferably the bonus effects expand from the bonus level. For example a spell at ML66+ might go from being touch to (SI-5) yards. That way even if the spell went to ML feet it would be (ML-60) feet starting near touch and growing from there.
A fireball that did SId6 ethereal damage but at ML80+ optionally doing (SI-6)d6 of physical damage instead.
Or Breath of Dhivu might do 1 point of real damage per die at ML80+, increasing to 2 points at ML100+, 3 points at ML120+ and real damage at ML130+. (Or at a higher complexity 60+, 80+ 100+ and 110+).
As a minimum I assume that the adjustment is at least half the level at witch the bonus effect appears: for a ML60+ bonus effect the effect might be based on (SI-3) or (ML-30) [A fireball that did SId6 ethereal damage but at ML60+ optionally doing (SI-3)d6 of physical damage instead].