If this is how terrible you want it to be, you might just as well remove it from the game entirely.
If you think that you would use it in that state, you might just as well not use a blitz ability to get the same experience.
I just did a few tests regarding the time it takes to kill all of those specials+elites in the psykanium.
I did not use a proper timer, but used the overall dps on the mutant as reference.
I hit the mutant once (to start the dps count on the mutant), then cleared everything else and finished off, killing the mutant last (to end the dps count on the mutant).
In this test, the dps of Assail + 4 stacks of SB on special/elite kill (~400 dps) was pretty much identical to a veteran using volley fire with an infantry lasgun (~400 dps) WITHOUT maniac dmg, and got beaten by a nerfed autopistol (500-600 dps depending on reloads) WITHOUT maniac dmg.
If i had maniac dmg on my guns, the veteran dps would have been quite a bit higher.
On top of that, the number of the psyker dps test is massively inflated, since all of these enemies are packed very closely together, allowing for all assail casts to hit 3 enemies, and for all of the soulblaze on kill effects to de some work.
In realistic scenarios, assail will often miss due to range (or require you to do the much slower right click cast). It will often hit trashmobs or not chain to anything. The soulblaze stacks would have basically no effect (on specials/elites) most of the time.
On top of that, if you use assail for trash clear, you will not have enough charges ready, in order to spam away at specials/elites.
Although assail will quickly kill small groups of trashmobs throughout the mission, those are not dangerous enemies and they could easily be dispatched in other ways.
While Assail probably offers more trash clear across the entire mission, than some (maybe even most) ammo using ranged weapons do, it is much weaker at clearing specials/elites, even within optimal range.
When it comes to taking down specials and elites, it is definitely not overpowered at all.
An other thing that people like to ignore, is that assail requires you to keep los and keep aiming at the target, until the shards arrive (which makes it pretty bad at longer ranges and if you are under pressure).
Guns do not have this issue, since they have incredibly fast projectiles (or might even use hitscan).
A good veteran can kill most specials, before the first assail shard even reaches the target.