BUZZ-Telefonica Deutschland: "O2 brand" weakness, RBC cuts to 'underperform'

** Shares (Berlin: DI6.BE - news) in German mobile operator fall 2 pct

** RBC cuts to "underperform" from "outperform", saying co's core "O2 brand" weakness base needs reinvestment

** "Incremental cost savings peaking in 2018 and revenues from core "O2 branded" subscribers falling will need to be addressed in face of pressure from convergence and family bundles from DT/Vodafone.", says RBC

** Broker adds that Deutsche Telekom (IOB: 0MPH.IL - news) /Vodafone are progressively increasing competition

** Stock among worst performers on German technology index and Stoxx 600