employ and fix the compensation of such additional staff personnel as is deemed necessary, without regard to the provisions of title 5, governing appointments in the competitive service, and without regard to chapter 51, and subchapter III of chapter 53 of such title relating to classification and General Schedule pay rates but at rates not in excess of the lowest rate for GS–15 of the General Schedule: Provided, however, That not more than 14 staff personnel at any one time may be employed and compensated at a rate not in excess of GS–15, step 10, of the General Schedule;