Okay, after beating the game once on normal, i'm going through it again  on VH. While leveling i'm trying to figure out what would be the best order to take the perks i've selected for the build. Was trying to decide between space ranger and keen observer as the next buy. So i decided to do some testing to see which one had more effect.

I'm on Dorado atm, using a silenced sniper rifle.

Crouched, not moving. I line up a shot on one of the Dunsany brother's heads to use him as a test dummy.

Fire a shot before buying a perk : 1509 damage.

Reload game, select space ranger.

Fire a shot:  1719 damage

Reload game, select Keen Observer

Fire a shot: 1509 damage (the same as not having Keen Observer)

So it seems like Keen Observer is not doing anything insofar as boosting weakspot damage.

 

(additional note, i'm using the monocular target-finder helmet, and I did some similar testing with it on and off and it sometimes seems not to add any damage. I get the same damage wearing it as I do without it when that should not be the case if it's giving any bonus to weakspot hits.)

Just did another test. 2260 damage without monocular target finder on, 2273 with it on. It made a difference of 13 points. Was a fairly close range shot so I assume the minimum benefit was applying... but it seems like it's adding 10% of the weapon's base damage at the tail end after all the sneak stuff rather than a 10% boost to the overall hit. Though.. adding 13 damage when the shot is already hitting for 2260 seems a bit on the pointless side.