Again, I'm not sure if I'm following your point. If a good tier 2 / 3 coach is more akin to a skills coach, why would turning a tier three team in to a tier two team be an indicator of guaranteed success with a team where, as you say this job isn't necessary. I agree with your contention that the skill sets required are different, but don't understand why this should translate in to having the different skill set required by a tier one job.
Rugby aside, on a common sense level, the diminishing returns point is the more significant one. Given an healthy, untrained, motivated individual, I'd feel confident in doubling their powerlifting total. Give me a competitive powerlifter and I wouldn't havethe first clue about how to add 10% to their lifts.
I think your last paragraph is trying to say that a tier one coach should be a good fit for the job that he's being looked at for. There's definately something in this for most jobs given that pretty much every nation has their limitations somewhere, but I don't really see how this is relevant to tier 3 success being relevant to the potential for tier 1 success.