I've been working slowly on putting STi suspension into my car. The short answer: Lots of work. Tons.
The long answer:
You'll need a custom attachment point for the front sway bar, different end links, you'll need to drill new holes in the strut tower of your SVX, and enlarge the big hole in the center. You'll also need to machine a couple mm off of the knuckle flanges front and rear. I'm not sure how much drilling will be needed for the rear strut mounts, but it will probably be much like the front. Also, the STi rear strut is longer than ours, so you will need either drop springs or to run adjustable coilovers to keep the rear end from sticking up about 1" higher than stock. By the time you're done you won't be able to return to OEM SVX struts without getting another set of knuckles.
Have fun!
I hope to go through with this at some point on my car. First I plan on buying a cheapo set of coilovers (Megan Racing perhaps) and making everything work. Once that's done I'll sell that set and go for something fancy.