It not exactly the question being asked but it might be a good thing to delay even if you don't have insanely high Lingering Insanity stacks when there is a fight mechanic that will force you out of the Void Form anyway. Even with 40stacks if you can only do damage for 10 or 15 seconds you don't get a lot of damage on this phase but lose a fair amount once you can DPS again the boss as you have low LI stacks and have to build 100 insanity anyway as opposed to directly entering VF.