I think it's essential to take good players out of the level they are currently in and see how they respond to a higher standard, it can sometimes be surprising.
I am mindful of a certain country area in the 70's where two clubs each had an exceptional centre, with A a more dominant player at club level than B. At zone level the scenario was the same, but when they then stepped up to NSW Country level A suddenly lost his dominance, and never really progressed beyond that. B, on the other hand, still looked good at that level, as he did playing for NSW, and then with the Wallabies until injury cut his career short.
The message here is clearly the big frog in the little pond has to be exposed to a bigger pond to see how he will respond.