so will a charged sinnite dagger instakill someone with full HP and sinnite armor? Or is it geared to be more like a fight starter so the other person is severely disadvantaged
A charged sinnite dagger hit from behind deals 120 damage. Sinnite armor reduces damage taken by 50%, so that would deal 60 damage. With the Sarti bonus to dagger damage, that's increased to 132 damage, or 66 damage after armor reduction. I have a pretty good idea of what feels fair with regards to stealth/backstabs from stalker DM and that seems about where I'd want it.
After a successful backstab against an armored target it would generally be a good idea to switch to a sword to finish them off, as daggers are a bit unwieldy to use outside of an ambush situation. The HP, positioning, and surprise advantages granted by the dagger hit from stealth should definitely tilt the fight in favor of the ambusher. If that turns out to not be the case, stealth attacks could grant a damage bonus.
I definitely prefer to throw numbers at things and see how they work in an actual situation than to try and guess at everything else that could be playing into the situation.