Quickest I've seen is sustained 17kts with the kite up on a reach from one end of my local puddle to the other. I can get 5-10 second plots at around 19; I have a 20.2 kts plot (around 5 seconds) when bearing off in massive wind and then staying out on the wire on a broadish reach instead of coming in to pop the kite.

I believe that top speeds will be without the kite as there is a lot of drag in the Spi; But the angles will be different between 1 and 2 up as the weight on the wire is different; 2 up will be on fairly tight reaches as you need more power; single handed top speeds will be broader reaching.