What is the best performance modification for a Ford Focus ST?

I recently purchased a Ford Focus ST and I'm looking to improve its performance. I want to know what modifications would give me the biggest bang for my buck. Any recommendations?

In my experience, installing a cold air intake and a performance exhaust system can significantly improve the power and handling of a Ford Focus ST. You'll also want to consider upgrading the suspension and adding a tune to optimize the engine's performance.

I would recommend starting with a high-flow air filter and a cat-back exhaust system. These simple and affordable modifications can provide a noticeable increase in horsepower and torque. You can also consider a chip or programmer to further enhance your vehicle's performance.

For the best performance gains, I would suggest investing in a turbo upgrade and a custom tune. This may be more expensive, but it will give you the biggest improvements in horsepower and torque. You can also add a limited slip differential for better handling and cornering.