The biggest driving force towards my choice of seat replacement has been my fiance. Connnnssstttaaannntt complaints on trips of more than 20 minutes. Theyre so stiff and offer very little support. You definitely can replace the foam, all you need is the foam, a pair of hog ring pliers and probably 1.5 hours worth of tedious patience.
How much better would it get? I'm not sure. It would probably be more comfortable.. but cost effectively, you may be better off trying to find some GD impreza seats.