they all have the same engine (except the big engine in the one Legacy Outback trimline, and the turbo in the WRX) but there does seem to be a difference Impreza to Legacy in actual use.

My uncle and I have each had both Legacies and Imprezas, and we've each gotten better mileage in the Imps. Maybe cuz we haul more in the Legacies, maybe cuz they're a heavier car, I dunno.

I still want the Subaru diesel boxer engine... gimme gimme!