I've only got 150 div or so into my sorc and it feels invincible with about 15k ES. Sorc does make ES quite a bit stronger than anyone else though. And to be fair I've barely needed to upgrade anything since like the middle of week 2. No idea what the gear would cost now, but with my dogshit luck I never even came close to being able to buy a Mageblood or anything like that, and no fancy lineage gems like Uul-Netol's or Her Declaration, just a ~25 div Vorana's.
I highly doubt expedition mobs were intended to be as insane as they are though. There's no way they won't at least tune the extreme outliers. The Fubgun video isn't the greatest example since that wasn't his original build that he dumped like 10 mirrors into, and the "kill anything with Arctic Armour" gimmick is almost certainly going to get nerfed to oblivion too, but some of those rares are going to be unkillable by the vast majority of builds. No other content juiced to the max is even remotely close.
Yeah, it's a really good build. When I said I preferred the other builds I tried (grim pillar totem oracle and glacial bolt gemling) it was strictly because the main djinn ability doesn't aim well with controller. It's great for clearing trash in normal maps, you can just hold down the button and controller auto-aim takes care of killing everything. The only downside is movement speed, and there's no way to get any more than I already have except for Mageblood.
For bosses it's weird. Sometimes it will delete them in seconds as expected, other times the targeting doesn't work and you end up blowing all your mana without hitting anything at all. It doesn't happen with m/k since you have to manually target everything.