Standard WRX hardly has any lag! Sell 2.0L 03 WRX, buy 2.5L 06 WRX. Bigger engine, same size turbo.
But, what TJ said, stroker kit or 2.5 conversion.
Or just change down gears

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i