5 mmo per kill I think it would be exaggerated, assuming you do at least 25 kills it is 125 ammo more than half I would be reimbursing you, now if I remember that before the assa and neme gave 1 ammo per kill, that is more acceptable and I don't think that is a bad idea, since as you highlight sometimes one is very close to 0 ammo after buying the mod.
and the nemesis I think is fine, people don't buy it as much just because it's hard to use compared to the assassin