If I recall, back in SOTIS days, when activated, Spell Storm was also cast on nearby allies. By that frame of reference, it's already been nerfed to 33% of it's original power.
That being said, a 50% nerf to effectiveness is rather stark, especially without compensation. At the same time, full negation is an unfun mechanic. Maybe Spell Resist that diminishes over time to smooth out the effectivity curve so that well-timed Spell Storms are both rewarding and well-respected?
Could be worth looking at.
I love your avatar.