I tested the Eye for an Eye card yesterday, and again with fisherman's help this morning.
The tool tip says : headshots restore some deadeye. In testing though it needs to be a headshot kill. Headshotting someone using the hat protection or slow and steady gives zero deadeye without the kill
But when you get the kill it gives alot, approximately double what a normal kill gives. Using level 3 restores 25% of the outer ring.
So, using very rough math, if you are using lvl 3 eye for an eye every headshot kill gives 6-7 seconds of deadeye time, if you are using level 3 unblinking eye as well, the same kill will yield 9-10 seconds of deadeye time.
Conclusion for me is that these cards are quite good for most situations, exceptions only being if you use alot of shotguns, bows, or melee (name your weapon modes in particular) or are using tonics frequently. In that case other cards would make more sense.
Hopefully that helps someone.