I was very close to purchasing a 15 STI a few months ago. I could get by the VERY outdated powertrain, and all the stuff we know about that, given it would be new, and have a 5/60k powertrain warranty, and I'd have left that stock. I couldn't get by the gas mileage, and insurance costs.
Simply too many compromises in that vehicle for me to pull the trigger. I was only willing to overlook so many downfalls.
It wouldn't even have to be at 30mpg, something like 26-28 would be totally fine, and even 350hp in a modern Direct injection engine with a turbo that spools a bit earlier and holds longer, etc. I would be VERY interested in that car.