Up and down stab actually hit more to the right than the mesh shows. Just keep strafing to the right and spam, it's hillarious how effective it is, since if your enemy doesn't make direct hit (which doesn't happen often, cause of slow turning speed and uncoordination of mesh and hitboxes) you will always hit before them. Or if you're not that spam happy, wait for enemy to try spinstab to his right, then start strafing to the right and stab him, you will always hit before them 
Yup, this is true. One of the worst parts of this game is the up-stab. They pretty much never glance, are almost instant, and don't even seem to take momentum into effect for damage. The animation is completely inconsistent with the rest of the game, and I still do not understand why bayonets do more damage than swords. 
I can't seem to find anyone who likes it... I wonder why its still implemented?
Upper stab actually takes momentum into account, but even with sweetspot damage reduction, it does a lot of damage even at 0 range, because it will headhit pretty much always. Sweetspots seem to decrease the damage done in percentages rather than making constant limit on it (which works nicely with armour, but here strange things happen).
As I posted several times, higher ping people (60+) will get almost no time to react at 0 range overheads which will do lot of damage.
Upper stab damage is inherently lower than lower stab but all head hit damage gets 2x multiplier after armour reduction and absorption is done (IIRC), but since there is no armour here it pretty much means it does double damage than its default value (which means it does a lot more damage than lower stab). Unfortunately, you can't test this on dummies since they don't have head hitbox.
Mostly upper stab is ridiculous because it will 1 hit kill anything that's not line inf no matter of movement.