The announcement, if true, would indicate a significant escalation in Iran’s campaign to destabilize its Sunni neighbors. Bahrain is particularly vulnerable because it’s a majority-Shi’a country that’s geographically close to Iran, ruled by a Sunni minority. During 2011, large-scale protests wracked the country, and were only with difficulty (and a blind eye from the U.S., which stations the 5th Fleet in Bahrain) repressed.
But for the same reason, Bahrain has a larger than usual incentive to be lying about this discovery: such an accusation gives important pretext to crackdowns the establishment feels are necessary to survive. As the Reuters piece implies, Bahrain has something of a history of crying wolf—if not outright fabrication/false-flag operations—in regard to Iranian terrorist threats.
Either way, though—as an escalation, or as a more bold, clear accusation than usual—this is big. The Sunni-Shia ethnic tensions in the Gulf are rising again, just as Assad starts to totter and the Iranian negotiations draw to a climax. Bahrain’s weakness, and the results of this claim, represent just another way the regional fires could spread.