As has been evident in this feedback forum and the Discord, Psyker’s Brain Burst does not perform well in higher difficulties (Heresy+). This is because its damage doesn’t scale, leading to enemies that were previously one-shots becoming two shots (Hound, Mauler, Rager), and other elites & specials requiring more BBs as well.
At the same time, letting BB not overload you if cast from 97% makes it so it becomes as spammy as it was in the closed beta, since it is absolutely trivial to quell for 0.5 seconds after each cast to chain BBs. This seems like something you wanted to avoid.
I think a simple solution to both of these issues is to buff BB damage considerably (so at the very least it one shots Hounds, Maulers and Ragers on Damnation) while also making the overload breakpoint lower, even something like 80%.
Brain Burst becomes more efficient while becoming less spammable. Easy.
*Brain Burst also has other issues such as building half of its Peril before you cast, which is infuriating when allies kill your target, or how you solely rely on BB for gaining Warp Charges, and more, but I won’t get into those in this post.