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I'm not sure I agree with that 100%. It's important, sure, but it's not the only thing, or even the most important thing.
Casey Stoner won the Aprilia 250 Cup in Australia and then left for Europe at 14. He never raced in the Australian Superbike Championship (now called Formula Xtreme). Troy Bayliss did race in that championship, however, but never won it. His big break was getting a one-off ride in the 250cc class at the Australian GP and finishing in 6th on a horrible bike. After that he got an offer to go to the BSB.
Troy Corser ran Formula Xtreme, though, and he competed in the AMA series. Then he went to Europe for WSBK...
Wayne Gardner's son Remy is competing in Moto2 or Moto3 in Europe, not in Australia. Mick Doohan's son Jack is trying for a F1 ride so he's on four wheels and not two...