[baxter001]

I'm not sure it scales like that for that long, sure there are benefits of familiarity with a language or system, the major changes in productivity I've seen come about through cross-skilling you can only expect the majority of devs or BAs or whatever to reach a certain level of competence when constrained to their core roles, encourage them to develop skills outside their usual BAU activities and the mutants you get back are an asset both in individual productivity and team redundancy.