Not that much of a nerf tbh, most of the time you were wasting your insanity gains by the time the stacks got high enough for you to start worrying about it anyway. The change just made it so that we need to be a bit more mindful over our ability use (not spamming SW on CD) when we reach the higher stacks, void bolt swapping will become more punishing on mistakes, etc. Not to mention the fact that we will be getting a 2nd 'Pause' to our degradation with the artifact via void torrent.
I'm guessing (and this is just a guess) that it'll end up being about a 15%-25% nerf to the damage you do in STM. It's a 16% nerf to insanity gains. Without doing any actual math, I'm guessing that'll end up being something like a 10-15% loss in stacks (your Dispersion buffs it 6 stacks, and 6 more if you make it to 96 stacks, and those aren't affected by the nerf). That said, that loss in stacks is at the highest damage part of STM since you are casting the most spells and since your filler is no longer in the rotation, so it has more impact than losing the DPS at the front of STM.
The nerf just made it so that making minor errors in your rotation become much more punishing. Before the hotfix a misstep in your rotation wouldn't always get you killed at high Voidform stacks. Not the case anymore. Simply makes it more rewarding for players who have a better grasp of the spec's playstyle.