Is hydrophobic down the same as waterproof down?
No. It resists moisture but will still wet out in heavy rain. You’ll still need a shell jacket.
Does hydrophobic down last as long as regular down?
Yes, but the water-repellent treatment may wear off after many washes.
Is it worth paying extra?
If you’re often out in damp, unpredictable conditions, yes. For dry, cold alpine environments, untreated down works just as well.
What makes the most difference: down treatment or jacket construction?
To stay fully dry, construction will have the biggest impact. A water repellent shell will be better at keeping you dry than a DWR-coated down. Hydrophobic down also has a 'breaking point' where it gets too wet and saturated.