Very poor example; the needs of decoding higher resolution video are nothing like displaying a UI. It would require a little more CPU time, but nothing like the roughly 4x or so 480p vs 1080p difference.
However, apparently, DX/K2 have half the ram (128 vs 256), which _would_ affect updating, for the browser at least. I heard someone messed around and ranthe K3 firmware on a K2, but it was very unstable due to the lack of ram. Besides that, there is no reason why they could not run it; same CPU, hardware is basically the same with the exception of the RAM.
They could likey make it work if they didn't keep the kindle "environment" (aka all that java shit) loaded, and just launched the browser - it accounts for a very large percentage of the ram on the device. When browser closes for whatever reason, restart the environment. Presto, the browser will have as much RAM available as it does on the K3 series devices.