The average premium for an Obamacare benchmark plan will rise slightly in 2015 and increase about 6 percent a year during the rest of this decade, the nonpartisan Congressional Budget Office said on Monday,BusinessInsurance.com reported.  The forecast suggests that insurance premium hikes under Obamacare may not be as severe as predicted by Republican opponents and some health insurers. The average annual premium of the second-lowest-cost "silver" plan would increase by only about 2.6 percent to $3,900 in 2015 from the current $3,800, the report said. The cost would rise more rapidly to $4,400 in 2016 and about $6,900 in 2024.