The big factor is cost and market placement. Add a turbo and you compete with other cars in the market. More power, bigger brakes all adds top the bottom line and then it looses that competitive sales edge. Like the car? Want more power? It's a canvas. Build it DIY style. Not too many stock WRX's around....

This is the car that Honda should have built with the outgoing F20C / F22C.
The massively overpriced S2K at around $80k (2009 figures) on the road is proof you can price yourself out of the game.

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MY00 WRX. 89 Prelude. 99 CRV