There are 3 different sizes there. 6x4, 10x5 and 12x6. The 7th page of the Pariah Nexus mission pack Tournament companion does an excellent job explaining it all!
I mean all it does for deathgaurd is to make 66% chance to hit to 50% still a nerf but not as abysmal as some gaurd stuff now hitting on 6’s if it moved
I don't know, I went 100/100/100 (the last game ended early because I tabled him, but I would have gone 100) with Tyrranids in a RT last weekend, so it can be done