Been trying to figure out the answer my own question above, but can't reach a high level of certainty.
I _think_ this is the first bit;
All players hit for twice the damage with head shots, whenever they land one. So a default, across the board, 200% damage 'bonus' for head shots.
Additionally, I think, (and meganoth just wrote confirming) that "sneak" shots add a 200% bonus damage to hits anywhere on an unsuspecting zombie (they can't be aggrod/targeting a player) while the player is also crouched. But this only applies to the first hit.
Big question on this is does the 'sneak-shot' bonus get stacked with the general 'head-shot' bonus for that first head shot?
(using Perception L5 + Dead Eye L3 head-shots are either 240% or 270%; does sneak add another 200% for 440% or 470%?)
Ok, thanks for this
@meganoth, I was grinding to the same conclusions for those two bits.
Bits I'm uncertain of are these;
1) Does the 'Tree' bonus, 'Percetion L5' = "Deal 240% head shot damage" actually only result in head shots hitting for 240% of base damage?
--- can't really imagine the tree bonus being additive w base head shot bonus or = 440% : this being seperate than if sneak shot 200% is additive ---
2) Does a perk like 'Dead Eye's additional, general bonus, "Deal 30% more damage" (at DE L3) stack on top of head shot bonuses?
--- again I'm leaning towards it not being additive, but that's just my guess ---
On a seperate tangent I'm thinking that maybe the 'speccd into' parts for Aiming & Reloading times might be more subjectively impactful than the simple numbers may suggest. By that I'm imagining getting into a 'oh he77' moment with several zeds. While the damage buff from speccing might mean it's only taking 3 rounds instead of 4 to drop a zed, that horrible period of reloading whilst getting pounded on is hard to forget, so maybe we tend to notice even fairly small improvements to that one bit leave a mark?
No idea really, just pondering my own 'belief' that using non-specced weapons early on isn't the best idea. Yet I'll qualify that by saying I do look at the stats & let those guide my choice, along with available ammo of course. (I was using a non-specced AK early on in current playthrough as it not only did more per round damage than the 'speccd' Pistol I'd looted, I had similar amounts of ammo for each and the AK both out ranged the pistol and had over twice the mag capacity.)