The Subaru cup cars in Norway use E85. They are not Gr N, but a re-mapped original engine. They are 280 hp and 580 nm.
But with full Gr N tuning, 33 mm restrictor and E85 re-map they should be potent. But not sure how much they take out on a full Gr N petrol today ?
But on a normal road car, they say E85 give 5-20% increase in hp using e85.
http://en.wikipedia.org/wiki/E85_in_standard_engines