[MENTION=6795602]FrogReaver[/MENTION], Serious reply, not an "instant naysayer". I'm commenting to improve your calculations so we can get a clear view. I see two things I don't think were taken into consideration and I would be interested in how much or how little they impact the end results.
First issue is that overkill is about damage wasted. The calculations shown do not differentiate for the twice-attacker between if the kill is done by the first attack or the second attack. Because if done with the first attack, then there is an additional attack that can be used to start damaging the next. If that's ignored, that's being treated as "overkill" (wasted") damage just like any extra done by the killing blow, but it actual play that is the opposite of overkill, that's damage that can be redirected to another target.
Perhaps a better way would be to see how many can be killed in 5 rounds. Or if you want to keep it on killing, then assume 5 opponents and count how many are (statistically) alive each round to make attacks against you.
Another issue that isn't being taken into consideration is that damage isn't static, and that aspect not being modeled in your calculations has an impact on the result.
If the single attack was 5-11 damage (avg 8) and the two attacks were 2-6 damage (avg 4), I think you'd find overkill matters a lot more. Because 1 hit from the first would always kill, but 40% of the time one hit from the second would also kill. The flip side is that 4% of the time it takes three attacks from the double-attack character, when a 2, 2, and anything are the damage rolls. But that's a lot less likely than that 40% when the overkill isn't important.
This won't be as large a change once you reach higher levels and HPs that greatly exceed damage of a single blow, but it does still have an impact.