Impale effectiveness with culling strike

1. Is it correct that impale does the most damage near the end of enemy life?
2. Since culling strike allows skipping that part of the kill, does that mean impale and culling strike are a bad combo?
3. I'm looking at my final ascendancy being Overwhelm (crit) or Headsman (20% culling strike)
4. My build already has a lot of impale to fight bosses better. But this could be switched to bleed.
5. I'm already using Assassins Mark and plan to use cast on damage taken to cast it after upgrading the gems to 20% quality. Then I'll be able to get 10% culling strike against bosses with Marked for Death.
6. It seems to me Overwhelm will be much better, am I missing something?

Impale does the most damage once you are X number of hits into the guy you are hitting, with X being your impale limit (base is 5, Master of Metal from Champion takes it to 6, there's also a watchers eye that adds 2). If playing impale, you want to juice up the impale effect as much as possible. This is not counter to also playing Headsman (which is a great ability).

Yes, even with an impale build it's a great passive to have, but so is Overwhelm. Lets say I manage to get 8 impales and a lot of increased impale effectives, then my dps against a boss will be at least double. With Headsman I'll skip the last 20%, whereas with Overwhelm I'll get much higher crit damage. So non-crit impales will get overwritten with crit impales.

Which is a relevant mechanism with Cyclone. Because with Cyclone the first hit always does half the damage. So the impale mastery 20% increased effect against non-impaled enemies is really bad, that impale is likely going to be for half damage.

Overwhelm's value heavily depends on the weapon you use. That 8% base overwrites your weapon's crit chance. So if you have a weapon with more than 8% (due to say some mods that give crit chance also), you will end up losing crit. And if you have a weapon close to 8%, you will get nearly nothing. I've played a lot of cyclone back in the day and I can tell you Overwhelm is much better on paper than in actual practice.

Mathematically, looping off the last 20% is basically 25% more damage. But it gets better than that--some bosses have phases where they don't regain full life on the last phase. They are also at their nastiest on their last phase (i.e. they pull out more moves). In those cases, you aren't just lopping off the last 20%, you're lopping off the HARDEST 20% of the fight.

@TemijnGold Thanks, hadn't considered that. I'm using Starforge, a 2h sword. It's base crit is 5% and it has no increased crit on the weapon.