I have recently started competing in autocross. Because of some after-market wheels/tires, which I bought before I started autocross, I can't compete in the Stock class. The class I'm in, Street Touring, mainly allows for suspension upgrades.
I'm not ready yet to put thousands of dollars in to my car, but I would like to make whatever cheap and effective upgrades I can that will be legal in my class, and leave my car suitable as a daily driver.
The logic behind starting with wheels/tires is clear to me: the benefits of other modifications won't make much difference if your tires can't transmit the force to the road. For example, an extra 100hp from a supercharger won't help much if you're trying to channel it through crumby stock tires and narrow wheels. A decent set of wheels and tires is relatively cheap and will have a big impact on handling.
When it comes to suspension however, I'm not sure how to decide what to upgrade first. One thing I'm considering is a strut tower brace, since they only cost on the order of $100 and is easy to install myself. There are also kits that just upgrade springs, or just upgrade sway bars, just suspension bushings, or full suspension upgrades.
My question is: Will there be a benefit to a STB when I still have stock struts/springs/alignment? If not, what suspension component should I upgrade first? If I think that I might, in a year or two, want to install a full suspension kit, does it make sense to spend money right now on a just-springs upgrade?
In case it makes a difference, I'm driving a 2008 Mazda 3s.