I've seen a couple of reports (on other builds, it's nothing Slice specific) where people aren't seeing 8.0.0 in the lists of available manual updates. This can only be an issue with mirror servers as the JSON file queried by the settings add-on is 100% correct on our servers.
Agree - I have seen this on the Pi versions also.
What I found was that if you enabled 'Show Custom Channels' and then disabled again then 8.0.0 showed up.