** Telecom operator T-Mobile's TMUS.O shares fall 1.2% to $208 premarket

** Oppenheimer downgrades rating to "perform" from "outperform"; maintains PT of $300, representing a 42.5% upside to the stock's last close

** TMUS will "have a difficult time beating subscriber and FCF estimates after a decade of outsized share gains and margin expansion"- brokerage

** Competitor Comcast CMCSA.O has aggressively lowered broadband prices, and Verizon VZ.N likely to increase handset promotions and content bundles, leading to intensified competition and driving TMUS to eventually increase prices and slow share gains - brokerage

** Avg rating of 31 anaysts is "buy," their median PT is $270 - data compiled by LSEG

** Up to last close, stock down 4.6% this year

(Reporting by Anhata Rooprai in Bengaluru)

((Anhata.Rooprai@thomsonreuters.com))