"[T]he election is among the most difficult to calculate in the defined contribution arena, requiring contribution data for the entire working career of an individual employee.... 15-year catch-up calculations are one of the primary issues identified in IRS audits, and lack of compliance appears to be widespread; and ... legitimate utilization of the election is often low ... [M]ore and more plan sponsors who previously permitted the 15-year catch-up election have simply decided to eliminate it ... [It] is indeed an elective, and not a mandatory, plan provision, so it may be eliminated via plan amendment[.]"