I have 2 Imprezas with a combined mileage of 285k both on original turbos. Not worth worrying about. Parts on cars wear out and need replacing but Subaru turbos are not a known weakness. The little TD04s are cheap as chips anyway.
My 2003 2.0xt has averaged a bit over 30mpg over the last 3 years. The economy doesn't seem to vary all that much - I have seen a range between 26mpg (cold/short trips) to 34mpg (longer A road journeys).