In both the front of the rear of the GD sedan, there is a bulkhead immediately in front of or behind the strut towers, so IMHO ST bars have minimal value so those should be last on your list.
Springs are wound steel wire. Buy the cheapest ones with the specs you want from a vendor/manufacturer you trust.
As I understand, the legal ALK bushing is a pain in the ass and probably not worth the effort. If money is no object, go for it, but a sway bar would probably have more effect.
A serious front bar may and maybe an adjustable rear bar be an even better choice than a rear believe it or not with streetable springs. Minimizing front roll is key to maintaining negative camber.
http://forums.nasioc.com/forums/showthread.php?t=719115
For the rear bar as long as it came with upgraded rear mounts. Like springs, rear bar is a rear bar, but all endlinks are not created equally. My weapon of choice for the rear is:
http://www.poltec.us/products/subaru..._end_links.htm
The adjustable lets you eliminate or add preload since you can't corner balance without coilovers. If you go with a smaller front bar, make sure it has adjustable end links.
And don't the STIs already come with teh group N tops?