Front and rear sway bars.

22mm fixed front sway bar and 24mm adjustable rear sway bar as well as heavy duty links. Even if you don't have coilovers, this makes a huge difference.

Then chuck in a good wheel alignment. And some really good quality tyres. Once you drive on high quality rubber, you'll never ever want to touch rubbish tyres again.

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RS Liberty - Because WRX's are only good for parts...