Crossfire in Street Touring seems to fall with STU. It is not officially placed in Street Touring class, and my regional club and I have been trying this out. There's an argument for STX, but it is weak.

I'm currently trading with this fully ST prepped 350z with a good driver at the wheel, and I'm currently only in ST due to brake rotor changes. However, this weekend I am going to begin an experiment (installing today) with the firmer SRT6 springs with first test courses the first weekend of June.

Keep in mind, that if you choose a suspension upgrade which moves to "true coilover shock" design, then the car is no longer Street / ST (I think it is the same for SP as well) class legal.
Also, if you want to run in ST (I really enjoy the competition at this level in my area), then you'll need to compromise on the adjustable linkage to either only the top arms or only the bottom (my research recommends the top arms be adjustable). That should be plenty though, as you can use the top two arms as a camber controller.

So your upgrades depend on if you want to build to a class, or build more for maximum fun / performance! Are you planning on keeping it daily worthy?

Cheers!
Jack