I was able to get a friend up behind our waverunner, going only about 6-7 mph. He was on a Slingshot Hoverglide, with the standard Wing (H2) that's been the stock set up for a while. It was interesting, b/c at 6 mph I couldn't quite get the waverunner on plane, but he was riding just fine, maybe a tad slow. At 7-8 (hard to control that small of an increase in throttle) the waverunner would start to plane out, but it would get to be too fast for him. And I had to be careful not to back off the throttle too quickly or the rope would slack and he'd start to slow down. Which would result in a pretty good yank when it would tension again.
On a side note, when it came my turn, I couldn't get up on the board at all. It would just pull so hard that eventually I'd have to let go. After a few shots, we shifted over to a Mako King and I was an instant hero on that. lol Never having wakeboarded or waterskied before, I found this to be pretty fun, though much more quickly fatiguing to the arms/legs than kiting. I can kitefoil, so not really sure what I'm doing wrong to try and get up behind the waverunner. Pull is coming from too low, maybe?
Anyway, getting this back on topic...I can't imagine doing 14-15 mph behind the boat, not when 6-8 was our sweet spot. Good luck!