For the 99 SF - GC8
(classic) saloon struts work fine as do the wagon parts. The top hats on the rear Impreza struts will need to be swapped with your Forester parts. If you are buying s/h, it might be an idea to buy a new set of wagon springs for the rear, regardless of the source of the shockers, and these can be fitted when changing the top hats. 04 WRX struts are a direct replacement, no part swapping. You will probably still require some saggy butt spacers though, I'd suggest starting with the 1/2" item. I've linked you to the part (
04-07) on the PF site. I was running +1" spacers before I bought a new set of GT wagon springs, which now allow better handling of heavy loads and reducing of the spacer depth to 1/2". Also, if you require more than 1/2" of spacer, the top hats need to have the mounting studs removed
(PITA) and longer bolts fitted.
Some guys fitted STI struts and/or combinations of springs to the SF with mixed results, most of them reported the car to be too harsh and some went back to WRX parts, IIRC. Putting STI springs on WRX struts isn't a great idea because the damper will then be under damped, causing a firm and bouncy ride. In reverse, the ride will be equally harsh so the question of lowering springs needs to be approached carefully I suspect. However, that's not what you asked about but in case you had considered this point, that's my view. My 99, on GC8 shockers (
new rear springs, spacers,ARBs and links), acquired a very compliant ride over the rather harsh and wobbly OEM setup, and handles
much better too. The original ride was so hard that I was avoiding cats eyes and seams in the road.
If the ARBs haven't been changed from the OEM parts, this will also be necessary as they will be totally unable to cope with the lowered suspension geometry and the car will roll like a bag of water in the bends. Personally, I ended up with a Whiteline 22mm adjustable at the back on the softest setting, and a WL 20mm on the front although there was no apparent difference to the WRX 20mm item I had prior. If cash is an issue, s/h GC8/WRX parts (
shockers/ARBs) of those years will fit our cars and work fine. New links would probably be a good idea though.
Some negative camber on the back is good for handling and tyre clearance but not
quite so rubber friendly. I dialed in some extra negative camber on the front using the camber bolts (
-1.5 deg I think) which makes the steering feel much better.
Perhaps you could throw up a picture or two to help ascertain whether you are lowered already and if so, what needs doing. If someone else lowered the car before you bought it, you can't know whether it was done properly. A complete set of classic shockers should be available pretty cheaply from a breakers and new replacements are more plentiful than Forester dampers as well but if the back end is very soft/weak, the ARB should be looked at. The OEM part is 15mm diameter, and a waste of time.