Now we support setting instance level properties through the UI (from the Info Tab for any instance via the Manage icon on the custom properties table), we can solve setting custom speed for interfaces.
Setting an instance level property ActualSpeed and/or ActualSpeedUpstream (if different from downstream - if ActualSpeedUpstream is not set, and ActualSpeed is set, ActualSpeed will be used for both upstream and downstream) will override the Speed and BasicSpeed values, used for interface utilization calculation.
Another change - Speed and BasicSpeed are now set as discovered ILPs, rather than unchanging datapoints that were collected every collection cycle (minor efficiency gain).