If a GPO is controlling printer sharing, can regedit overrides be effective?

Generally, no. Group Policy Objects (GPOs) have precedence over local registry settings. While you *can* modify the registry locally, the GPO will likely revert the changes on the next policy refresh. To effectively override a GPO, you need to modify the GPO itself or use Resultant Set of Policy (RSoP) to understand why the GPO is applying the settings.