The maximum speed for an unmodified Prius on electric alone is 34 miles per hour or so. PHEV modified Prius can go much faster. Go to Priuschat, and there are several threads on PHEV-modified Prius.
20 HP could actually power the HCH in low-speed situations, but the battery pack would not last long if you put that kind of load on it. However, I still think you would need more than the 20 HP to make a usable PHEV out of an HCH.