Study by Minnesota State University says you can run a PriusI ( THS ) on E85 without any modifications, if you don't mind seeing a check engine light.
http://www.greenenergynetwork.com/me.../prius-e85.PDF

Highlights:

Cold starts on E85 produce slightly higher emissions, however emissions on a warm or hot engine are reduced to almost below detectable limits, more than compensating for the cold start. ( provided you do more than only cold starts every day! )

Dynomometer testing shows increased horsepower and torque:

89 Octane Gas = 65bhp @ 4500 RPM and 80 ft.-lb. torque

E85 ethanol = 77bhp @ 4700 RPM and 106 ft.-lb. torque

Since the Toyota traction control will not let the front wheels spin beyond 19 MPH with the rear wheels stationary, the traction control was disabled, as well as electric assist, so all horspower can be assured to have come from fuel only, and none came from the electric motor(s).