I am still tweaking the NN and KDW, but right now I have NN at 140 and KDW at 68. How closely it holds speed depends on a lot of things, chop, how heavy the rider is and how hard they pull, whether you're turning or not, amount of ballast.
I get speed control within .1 mile an hour when not pulling a rider, and control +/- .3 when pulling an rider under typical conditions with a sack in the trunk and one in the walkthrough. The system corrects as the rider cuts, so the speed at the time the rider hits the wake is usually exactly at the setpoint. It overshoots a bit when the rider is in the air, and corrects after the rider lands.
To be honest, I got tired of playing with the numbers and have just been riding a lot more. It doesn't matter who's driving, I have been letting my 11 yo pull me.
I'd rather ride behind and IO with PP than an Xstar with a typical driver.