The Lumia 830 belongs right where it is, nestled between the 600-range and the 1520. I owned a 920 and a 1520, and have gotten my hands on the 635 countless times (a friend has one, a work colleague has one, and they're littered all over our local Microsoft, AT&T, and T-Mobile stores). Performance was much closer to the 1520 and right on par with the 920 than it was the 635. People claiming it's more on par with the 635 clearly haven't spent time with all of the phones mentioned and are just b.s.'ing based on selective nitpicking of a spec sheet. Hell, even the PhoneDog review said the 830 was even more of a pleasure to use than the 930.
Add to that the combination of Glance Screen, wireless charging, top-notch aesthetics, premium build quality, built-in OIS, HAAC mics, expandable storage, removable battery, exchangable back covers... and you've got a package you won't find elsewhere, on any platform, let alone blowing the 635 out of the water.