Erm, let's see. . . here's what I can offer from my limited experience. . .
STi pinks and Group N topmounts are a safe choice, but you'd probably want to go ahead and pick up some damping-adjustable struts while you're at it. They'll be nice for going from street to track, and then you won't have to bother with disassembling your stock suspension assembly.
Getting adjustable top-mounts will mean pillowball joints, and that means your ride quality (comfort) will take a HUGE hit. The stock mounts are rubber for a very good reason.
But it will help stiffen up your handling a bit (less flexing) and add the camber adjustability. Just make sure you really want to be able to do that first. I'd say start with the Group N topmounts and then upgrade to camber plates if you decide you want more.
I don't know what the stock size is on the STi rear sway, but I'm sure an adjustable RSB would provide some benefits. Just remember that a bigger rear sway will make your car more tail-happy, and that's bad in the snow. As for the strut tower braces, there was a thread on the forum here somewhere discussing whether they're really even worth spending the money on over other possible suspension mods. They'll stop chassis flex to a point, but Imprezas are already pretty stiff.
Spring/strut/topmount replacements, sway bars, and strut tower braces are all safe for street touring and street prepared, I believe.