I just tested how Guerilla, Sneaking, and Invisibility stack with Backstabs.

Weapon: 35-58 Piercing damage poinard
Target: Charred Bone Idol (perhaps not the best target)

All attacks were backstabs.

From behind, no sneaking: 40 * 2 (Absorbed: 8?)
From behind, while invisible, no sneaking: 48 * 2 (Absorbed: 8?)
From behind, while sneaking: 140 * 2 (Absorbed: 30)
From behind, while sneaking, while invisible: 150 * 2 (Absorbed: 30)

I'll do some more tests, hopefully finding a squishier target.