The sportline springs are pretty low for daily driving, it might be better for both autocross and dd to get a set of decent coilovers. Your car should have a lot of choices for around $500 for a set and you get to choose your ride height and they simply work better.
Aside from driver, tires will be the best mod for you but it's also your daily driver so it's a tough choice for that. The new street tire to have for autox is supposedly the Bridgestone RE01R but their pricey. You can't go wrong with the Falken Azenis and the Hankook RS2 is very good and cheap.
You'll be in the STS class but we'll have to watch what type of braces you put on as certain types that mount differently will boot you to a higher class quickly.
Since your going to mod it, do it before the season starts if possible then once the season starts leave the car alone. The suspension and tires will help a ton but the biggest variable is the driver. Work on yourself and not changing the car will make "you" go faster and learn to drive better. When you do come out next year we'll get a good highly experienced driver to drive your car if you'd allow it and you'll see the difference the driver can make and you learn a ton from it, I did.