I have a 98 OBS. I replaced Both front and rear struts with brand new forester struts. Front switched out like a dream, had to transfer my original top hats from the old Stuts to fit the rears. Instant 2” lift on all fours with this. Then I put 40mm strut top spacers on top of those (about 1.6” I’d lift on all fours). After those I installed subtle solutions 2” trailing arm spacers. After that I swapped my front control arm bushings with forester bushings (about 1inch longer than Impreza ones). 

I need to do more but I want to know what’s necessary and what’s not. 

I think:

Steering shafts needs to be lengthened, trans needs to be dropped,                                  

I don’t know at all:

engine mounts/does it need to be dropped like the trans? 

Also do you guys know a bigger/thicker sway bar I can plug and play? I need more stability when turning. The forester sway bars are very close if not the same as Imprezas, the end links are the exact same.

http://www.subarubrat.com/hasseylift.htm

this should give you a rough idea of what a subframe lift involves, the main idea is the same on the newer cars. Check out subframe lift kits from places like paranoid fabrications to see what Impreza specific ones look like. 

As far as a sway bar goes my 03 OBS has a 99 OB sway bar that works great. Subframe lockdown bolts were the biggest upgrade, but you’re never gonna get rid of that feeling of being thrown out of your seat 

At 3.6 inches of strut lift I’d be worried about cv shafts wearing out