You gotta remember that modern engines have vastly superior breathing than our old motors do. The more air you can get into, and out of a motor, the more power it will make. The new abath fiat 500 makes 160 with a turbo 1400. (remember, thats a production car, and not too highly strung) that wouldn't be too difficult to acheive with a modern cylinder head design.
look at what people ring out of hayabusa engines!