Running the Optimizely module under Drupal 8, beta 3, both manual and automated testing showed no loss of functionality or run-time errors. This is the first time that migrating to a new release of D8 did not require any code changes to the module.
All I had to do was bump up its version number. The reason I made a new release of the module is to show that it has been tested and conforms to the new D8 beta. My current convention is to incorporate the D8 version as part of the module version, like so: 8.x-2.15-beta3.