The 15 surfs great if you plumb in some extra ballast in the rear storage compartment, you can pull the cooler out and throw some weight in there as well. We surfed it with about 500 extra pounds in the back corner, very nice, if it were my boat I would have added another 1000 or so to really beef it up.
I typically ride an X-1 with about 2500lbs of ballast so riding the stock ballast in an X-15 didnt blow my mind but it did the job just fine, I easily landed all my tricks. Again, if it were my boat I'd beef up the ballast.
Overall I'd say the X-15 surfs far better than an X-Star and an X-1 though I'd give the X-Star the edge when it comes to wakeboarding. I need to ride a sac'd out 15 to really compare it though, it looks like the potential is there just havent experienced it myself.