I never noticed a problem with it holding surf speed on my 2006 X45. I do think it may have helped a little to gently put the throttle down further about 5 seconds after the MC cruise engages. I know that you have to do that with throttle by cable systems to give the unit some 'slack' to work with. It shouldn't make a difference for an 06, as I think it is throttle by wire. But I think it helped for surfing and I KNOW it helped for wakeboarding. Often when turning, it would lose speed, but would not do it if the throttle was down.
And my X45 surfed well about 11 MPH. But I think most speedos get less accurate at those speeds, particularly when tipped sideways with lots of ballast. I GPS'd mine to be accurate at wakeboarding speeds.
Be excellent to one another.