About Wind Up time
[Killing Blow] reduces Heavy Attack wind up to 0.2. [Amalgam Organ Shatter] won't lower this number further.

About Critical Chance
Since innate crit chance is very low, there's no sense building it with more than [Sacrificial Steel].
If you choose to apply both Sacrificial mods, the max crit chance multiplier for heavy attacks is: Crit chance * 5,5

Full Sacrificial Set = 123,5% CC
[Sacrificial Steel] Solo = 102,6% CC

About Sinergies and Heavy Attack
Gunblade's Heavy Attacks shoots 9 pellets, each one with its own Status Chance, Crit Chance and damage.
Each pellet of [Vastilok]'s Heavy attack allows procing Status Effects, scaling damage with [Condition Overload] (Even beyond 30.5 patch) and applies [Shattering Impact] effect, stripping 54 armor on each full impact Heavy Attack.

[Corrupt Charge]'s +30 Initial Combo, grants a x2 damage multiplier

[Vastilok]'s Advantages
[Vastilok] has a 1.3 meter innate Punch Through.

Firing will stagger enemies within 4 meters radius.

Theoretically, [Vastilok]'s projectiles has 300m range, allowing to proc stat effects, strip armor and melee kill count at Primary weapon's fire range. Thus being useful for other game mechanics, like Primary/Secondary Dexterity Arcane, Arcane Primary Charger, [Saryn]'s Toxic Lash melee bonus damage, Companion's [Pack Leader] mod trigger, etc.