I think you set up the controls right. It looks like you had a thumb shifter set up on the right in the equivalent position to the throttle. Gears, throttle and front brake is too much for one hand when you are going fast, and something like a 14-28 6-speed would give you enough range for getting out of corners on what looked like a flattish course. You probably lost cadence by 25mph, but that Crystallite would have taken care of the rest!
It looks like a bike that was very well designed for the high speed manoeuverability that the course demanded. With the right type approvals there could be a fair demand for the production version - or even for a version with a legal power motor!
When Shimano first brought in STIs they were at 7-speed - they never made 6 speed ones, so you would have had trouble finding any! Agree they are the most overpriced product going, even second hand.