I checked my damage calculation spreadsheet for this question: With your equipment and RTW, the dmg of a Dual Shot at equal level vs. a Charr shaman is - min. 94 - max. 158 (no crit.) - crit. 216 (32% chance) Since a Charr shaman has 220 HP, you can never one-shot them with that setup.
With that 1% additional damage, the crit. damage with double-shot would be exactly the same, due to rounding.
Using Ignite Arrows with 11 in Wildernes Survival instead brings 34! additional damage. Your chance to kill the Shamans with one-shot is 32% then.
I highly recommend using IA for that reason (it's much more comfortable with a longbow though).
Fun-fact: even without any +x% vs. Charr mod, you would deal 220 damage and instantly kill them.
In case you wonder: you cannot one-shot the lvl8 Nec and Mes Charrs with Ignite Arrows. There are 4 points of damage missing (2 points with an +18% vs. Charr Mod).
A Golden Egg would increase the values with RTW to: - max. 162 (no crit.) => the additional 36 dmg with IA would still not be enough
- crit. 222 (34% chance) => BOOM! (Fun fact: with +16% vs. Charr you'd deal the exact same amount. Again, this is due to rounding. +15% vs. Charr will not work at all.)
A +18% vs. Charr mod would increase the crit. damage to 224.
Using Ignite Arrows, you can even one-shot the lvl8 Nec and Mes Charrs.
And some Specials in pre exists. So a +3 Marks Rune. Or a 25% bowmod.
Would this Change Something?
Thank you. Wasn't aware of Candy Corn.
I knew about the +3 Marksmanship runes being available but a +20% vs. Charr should be max., even in post.
Is your question the same beside that? --> Difference of +17% vs. +18% vs. +20% Charr mod? I would apreciate a question that can be answered with yes or no so I don't put too much work in unnecessary queries.