Notwithstanding any other provision of this chapter, an employer whose contribution rate is determined to be 3.5 percent or more may, on or before a computation date, elect to have his contribution rate thereafter computed on each computation date solely on the then most recent 5 calendar years’ experience. Such election by an employer shall be irrevocable. Provided, that in the event of such election, no rate shall be assigned lower than 2.7 percent at any time thereafter.