Fronts can also be an issue with sedan struts on a wagon - since the front track width is wider on sedans than wagons in the 02-07 years you will lose camber and can also (with inverted struts) experience some binding from the struts being put into bad geometry when using sedan struts on a wagon.
Saggy butt problems in the rear can be cured with a spacer. Spacers over 3/8" or so should really have extended studs added.
As for 04 STI sedan struts, don't do it. My '03 TS wagon has 04 STI sedan inverted struts all around, with a 1/2" Paranoid fabrications spacer in the rear (with extended studs) and 04 STI sedan tophats. I get binding and have lost camber in the front so I am swapping them out for '06 wagon KYB GR2s and 06 WRX wagon springs in a few weekends. Even the nicest 04 STI sedan struts are still ten years old, and although you can do the inserts, the binding and bobblehead associated with them (on top of the four hours I've spent tapping them for zerks and regreasing them) have really turned me off the idea. They're not undrivable - much better than the blown struts I had before - but they're awful. Maybe if I had sedan control arms...
I was also considering running Whiteline adjustable caster/camber tops to compensate for the front but it wouldn't solve the binding problems.
The reason is that the sedan clevis tab thingus stretches out further for the wider track, and putting it into a narrower wagon track forces the strut shaft in against the inside of the strut in a way that binds it:
As for tophats, 04+ tophats change the spring perch but keep the same stud pattern. If you can, angle for Group N tophats.
I'd be interested to hear what you think of the Swift springs. When I was still trying to build a performance WRX wagon instead of just keeping my daily driver TS on the road, I used Racecomp Engineering wagon-specific springs with a Koni insert into the stock wagon housings. They were sublime, but I assumed they would give me a lot more roll control than they did and I ended up messing with damping adjustment instead of just putting big-boy swaybars on it.
If you're looking for general GG wagon handling advice, I strongly recommend putting a rear strut tower brace in the hatch. The wagons are much wobblier than sedans back there and I actually had an incident where the hatch was rubbing against the bumper with sticky tires installed. If your classing can swing it, also get a Whiteline anti-lift kit.
Also on the 02-03 wagons, a heavy rear sway can tear out the stock rear swaybar mounts (they suck). Replace them with an 04+ rear swaybar mount to fix this (Subaru Genuine Parts has a kit that I've bought for the TS), or some aftermarket companies also sell their own version.
I got my 06 wagon KYBs in a kit from JustSuspension - once I figured out what part numbers I needed, JS had the best price and free shipping to the US.