We think we've identified the issue with the squeezed video and are working to get a fix released ASAP.
Have you figure out why the MyMedia channel no longer works with 2.8? The problem there isn't squeezed video, but no display at all; still, could the problem have to do with using Handbrake CLI to transcode to mp4? If there are recommend parameters to Handbrake CLI that could solve this problem, please let us know.
Also, can you confirm if blocking api.roku.com at the DNS level would be effective in allowing owners of the Roku devices to block the final 2.8 push if it still breaks MyMedia? The blocking seems to have some effect in that the Channel Store is no longer available, and nothing else seems to be affected, but of course I'm not going to try the manual update procedure in order to test it
I understand that Roku put some text in the EULA where you reserve the right to update the firmware as you please, but are you intent on enforcing it or will you provide a way to opt out? Anybody who opted out of course would no longer be eligible for support from developers or from Roku unless they install the latest firmware.
The best solution would be to fix 2.8 so that MyMedia and other local streaming channels still work, but I understand that might not be possible if MyMedia in fact is doing something wrong that only worked accidently with 2.7.