The main problem is that the damage you calculated doesn't take into effect the gear bonuses you get. Since it's flat damage, it benefits both from +dmg% and from focus. From what I understand, those work additively, not multiplicatively. So, let's say you have, endgame, 1000 focus. combine that with, say, 200% damage to your element of choice.
I don't know the exact numbers for Synergies' focus changes, but let's say that 1000 focus gives you 250% magic damage. That means we're looking at +550% to that electric damage, or multiplying the damage by 6.5, giving us 14398-43121.
Elemental %WDPS skills, and every other flat damage skill also scale from the same things, so Brands are nothing special in that regard (which was why I neglected to mention any kind of scaling, since flat damage scales from the least number of things, especially flat damage skills with internal cooldowns).
With +200% to electric damage, and +250% to magic damage, the combined Brand damage is actually 3 (1 for each brand; base amount) + 2.5 * 3 (magic damage boost) + 2 (+elemental damage of choice) = 12.5.
The combined Brand damage would then be: 27687.5 - 82925 (27.5k - 83k)
Each Brand crits individually, and with a 50% chance to crit with +250% bonus crit damage, the crit damage without +ele damage is: 81266.5 (81k; 50% of the time). While crit damage with +ele damage is: 127704.5 (128k; 50% of the time)
(Crit always hits for the upper limit)
However, it should also be noted that +% cast speed doesn't at all improve this build.
That being said though, it still outputs an insane amount of damage to the point where it would possibly overshadow the damage from the main skills. (or at least contribute to a significant portion of it)
But such a build utilizes all 3 brands, which means you would have to proc all 3 affixes (Burn, Freeze, Shocked).
There are several ways to do this:
PBolt (doesn't output much DPS on its own compared to Brands)
Wand Chaos + Shockbolts (WC is massively nerfed in Synergies)
%WDPS skills w/ a multi-element weapon (i.e. Arc Beam, or Shocking Orbs)
%WDPS skills w/ gears that apply affixes (e.g. Transcendent set, iirc)
Pets using the Freeze/Fireball spell
I feel that PBolt builds would be completely unplayable if Brands were to be severely nerfed.
Wand Chaos is completely underwhelming at the moment, and there would be no point discussing Brand interaction with WC when WC itself is so weak.
Arc Beam / Shocking Orbs with multi-element weapons is interesting, because Shocking Orbs from what I've gathered seems to be a pretty powerful skill, and with the Brands it looks like it could easily get out of hand.
As for Arc Beam, nope, never had much experience with it, never heard much about it either.
I'm not too familiar with most of the affix adding gear (i.e. % chance to Freeze/Burn/Shock, % chance to cast Thunder/Glacial Spike/Firestorm/Meteor Strike), except for the famous spell trigger socketables I guess.
Pet Scrolls could be an issue for Fire/Ice Brand since they would be too easily added to any build.
And as always, MP is a can of worms. Having someone else stack the affixes for you isn't too far of a stretch, I suppose.