I wouldn't be trying to squeeze out any more than 300hp from an 18 year old EJ205.
I'd be doing what i said earlier, going for a more modern, responsive, mid range orientated turbo and getting a retune. It'll feel faster, and will probably GO faster around a windy little track like Winton.

I personally wouldn't be going for eBay turbos, but there are people out there that have done, and have had success. I also know others that haven't.

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Balls.