To take a shot at this one...
You can fly a hull at low speeds, so the height of the hull isn't an indicator of speed.
A hull just out of the water or "kissing" the surface is likely fastest due to the reduced wetted surface of the second hull being out of the water. The more you heel, the more air spills from the sails. The more upright the rig... the more power, so healing can reduce power and speed if too much.
Top speed potential of a Hobie 16? Likely in the low to mid 20's (MPH).
Recorded on a 500m course:
Hobie 16, Hank Pauloo USA, Portland, UK, 16.90 kts
(
http://www.speedsailing.com/Background_records.htm)