Damage: 0 (0 x 0.05 = Miss Chance) + 19.35 (21.5 x 0.9 = Hit Chance) + 1.6 (32 x 0.05 = Crit Chance) = 20.95 damage.
...
First, though, I do want to note that the Rogue's damage is, undeniably, still twice what an average non-Striker will do. That was your original claim, remember - that this didn't happen prior to Essentials, and was a new thing. And it isn't true.
Let me get this straight.
Your claim is that average Strikers do twice the damage of average non-Strikers pre-Essentials.
You then pick one of the best first level Strikers in the game and optimize him heavily for damage and then re-iterate your claim that he does twice as much damage as "average" non-strikers.
It takes a little effort to get a non-Striker to do more than 10.5 average half DPR of your uber build, but then again, you are using one of the strongest DPR core builds. Rangers get nowhere near 21 DPR at first level with Twin Strike.
The DPR first level king for a Ranger is 14 DPR. 14. Not 21.
Warlocks are even lower for DPR.
Let's take a Fighter who gets CA from your Brutal Rogue (if the Brutal Rogue gets CA so easily by flanking, so should the Fighter).
Level 1 Dwarf Fighter
Str 20, Con 14
Dwarven Weapon Training plus Weapon Talent Fighter
Brash Strike Mordenkrad with Combat Advantage: +10 vs AC 15: 2d6b1+5+2+2
Damage: 0 (0 x 0.20 = Miss Chance) + 12.75 (17 x 0.75 = Hit Chance) + 1.05 (21 x 0.05 = Crit Chance) = 13.8 damage.
That's solidly more than half (66%) of your optimized Brutal Rogue you created.
Let's take a not too heavily optimized for damage Fighter.
Level 1 Human Fighter
Str 18
Weapon Focus and Weapon Talent Fighter
Reaping Strike Greatsword with Combat Advantage: +10 vs AC 15: d10+4+1
Damage: 0.8 (4 x 0.20 = Miss Chance) + 7.875 (10.5 x 0.75 = Hit Chance) + 0.75 (15 x 0.05 = Crit Chance) = 9.425 damage.
Typical Str. Average two handed weapon. Relatively weak first level damage feat.
And he still averaged almost half as much as your uber core build. He would easily be more than half of the damage of any Ranger. In fact, this average Fighter comes in higher than 75% of an average Ranger.
And what about extra damage for someone ignoring the Fighter's mark?
Even a Cleric with Implement Focus does more than half of the damage of a Ranger with a Superior Bow:
D8+5 * 60% + 13 * 5% = 6.35
D12 * 60% + 12 * 5% + D12 * 60% + 12 * 5% + 80% * 3.5 + 10% * 6 = 12.4
This does not even take into account multi-foe attacks like Controllers.
Yes, my claim is still valid. An average striker in pre-Essentials did not average twice as much damage as an average non-striker.