Tires can make a huge difference, but if you’re just using the car for commuting and aren’t doing any auto-cross or track driving, the OEM tires are probably fine.
My vote is for an ECU tuner on a forced induction car. When I had my Mazdaspeed6, the Cobb AccessPort was the single best modification that made the biggest difference to my daily enjoyment of the car. The basic stage1/93 octane map for an otherwise unmodified vehicle made a big difference, and then they had off-the-shelf maps as you modified your car to unlock its true potential, plus you can get it custom tuned based on your specific set up and goals. You can also check and clear trouble codes, data log the engine to diagnose problems, live monitor sensors and detune to safely run on lower octane gas.
You may not be able to make your own tunes without a lot of software and a dyno, but applying pre-made tunes to your car’s ECU is generally pretty easy. Something like a Cobb Accessport or OpenFlash tablet, that can save downloaded tunes and re-flash the car as often as you like, can allow you to change your power, fuel economy, or even throttle and transmission response on the fly.
Submitted by: jmjr07