I went for it. :)
Although I had a slightly different experience...
1. I signed up to the Nokia developer preview, downloaded the "current roll-back image"
2. Transferred it to an SD card via my computer,
3. Put sd card back in phone.
4. Rebooted phone to check for file before using recovery mode.
5. However, when the phone rebooted it found the file on sd card itself and offered to install it whilst phone was active (no need to use recovery mode).
6. I accepted and waited for the 100% to install, and then had to reboot.
7. Once rebooted I was on 8.1.0.
8. I only have April security update at present. (I suspect EE are blocking the May one).