I have a 920 and how I wish I could upgrade to the 830 (but I live in the US). First of all, the clock speed is slower but you add two cores compared to the 920, and it's newer architecture. I think the CPU is definitely an upgrade. Is it a proper 2-year upgrade? Maybe not, but that's a subjective decision. The GPU is, apparently, a step down but I don't game on my mobile phone (too much battery consumption). But, otherwise, I think it's a great update and here is my personal scorecard:
1) Screen size-- 830. It's bigger but not monstrously big like the 1520.
2) Resolution-- 920. Actually has a slightly higher resolution and, with the smaller screen, a higher dpi. Definitely an issue but outweighed by other considerations IMO
3) Size-- 830, by a wide margin. It's thinner, lighter, sleeker
4) Aesthetics-- 830. I have my eye on the orange back. Looks great. And if you tire of it you just switch.
5) Storage-- 830. Built-in storage is half of the 920 but the microSD expansion for the 830 sets it apart. Yes, it might be slower than internal storage but who cares? I'll store my maps, music, etc. on it. It doesn't need to be super quick
6) Battery-- 830. I don't care about battery life per se. I'm sure it will suck and I'll be disappointed, as I am with every smartphone. But it has a removable battery!! That means that you don't need to chuck the phone when the battery fails. No planned obsolescence here.
7) SoC-- I still think the 830 is the winner, FOR ME. Newer architecture, 2 extra cores, don't care about the GPU.
8) Software-- 830. This will presumably ship with Denim, which is just a dream of the distant future for me on my 920
Overall I think it would be a nice upgrade. Would a higher screen resolution and better SoC make it better? Of course. But I'm not convinced that those things are enough to torpedo the device, relative to the 920. Of course this is all an academic discussion since the 830 won't come to the US, but it's nice to think about.