Overkill T1 adds +1 to the kill tally whenever you kill an enemy. Overkill T2 adds an extra +1 to the kill tally for every 100 max levels you have, and T3 adds +1 every 50 max levels. Its only use is to run through Stages faster to grind Ascension Shards.
Sniper T2 doubles your damage if your crit chance is over 100%. No clue what Sniper T3 does exactly though...
Update: T2 Overkill stopped working for some reason. I stopped getting bonus kills at 100 max level. T3 does work, and I'm pretty sure it stacks additively with T1 and presumably T2 if it worked. At 191 max level, I should be getting 6, 1 for the initial kill, 1 for T1, 1 from T2, 3 from T3, but am only getting 5 kills. T4 gives you XP and Weapon chance for the overkills, but also costs 10m xp, so have fun unlocking it...
Edit: Also, I was wrong about Sniper T2. It activates whenever you crit, regardless of your crit chance. And it calculates weird, as it seems to be doing less than double. It looks to actually be 1.5x.
