The IS comes in two different models, IS250 and the IS350. the 350 making more horsepower of the two. The 2016 model tops off at 306Hp. The ISF is a completly different animal, on its last production year, this beast horsepower was 416. It gets that from its 5.0liter V8 engine. With its stiffer suspension compared to any of the IS's and a bunch of other features, the ISF is in a different class.