As Bryan said, not sure about the trans mount. I think they used used spacers for lift on the forester, so it should work without the spacers.
The forester driveshaft is different, the mount on the center bearing accounts for the lift. So you'll need an impreza specific manual driveshaft.
Also I think the pedal box is the same, but again, I don't have personal experience there. I'd just look how it mounts in the forester vs your car, I think it'll work.
All the clutch hydraulics will swap over, as will the flywheel/any 2.5 clutch kit for it.
Make sure you use the flywheel specific bolts, the ones for the flexplate on an auto are too short.
You'll also need to use the manual starter. The auto one is different... I can't remember why, the snout is either too long or too short.
You could just use all the lift blocks and suspension from the forester on your impreza, use the forester driveshaft and everything. The forester is basically an impreza chassis with a box on it. So they're more or less the exact same car in terms of bolt ons.