Many people report better handling with a disc in the rear, has to do with surface area and placement. I originally rode a disc and gave it up when I went from tubulars to clinchers. I regret it. I had ridden in 30mph winds on my disc. Not just that, but windy conditions is when you gain the most from aerodynamic wheels.
I figure if/when he ever qualifies for Kona, the $600 for a Trispoke will be a drop in the hat compared to the flight, hotel, race entry, qualifier and other associated costs.