The maximum distance scales with the orbital period as Kepler's Third Law so that relative to an Earth year, and the Earth-Sun distance, if the period of Hale-Bopp is about 4000 years or so, then (4000)^(2/3) = 240 Astronomical Units. The velocity ( circular) scales as the inverse square root of the distance, so for V(earth) = 30 km/s at 1 Astronomical Unit, for 240 Astronomical Units, the velocity is about 2 kilometers/sec. That would be my guess as to the velocity of Hale-Bopp at its farthest distance from the Sun to within a factor of two.