Soooo......I don't have a 242XE, and I don't surf so I can't really comment on the speed control issues......I do have a somewhat related question though, and it might help with some of this mystery.
Shouldn't water speed relative to the hull be more important for wake shape/size/push than GPS speed. GPS speed will give you speed over land, and not water speed over hull. If there is ANY current in the water, your speed over water will be different than speed over land, and thus give a wake with different characteristics? I would suspect if you are controlling speed over land (GPS) to 0.1mph precision (not accuracy, an important distinction), then wouldn't any small change in current upset the whole apple cart on your wave? This is a similar situation to pilots where air speed is more important than land speed. Fluid flow over the foil surface (air over wing) is more important than speed of craft referenced to land......If this is indeed the case, then shouldn't a paddle-wheel or other water speed sensor be more appropriate for control of wake properties? Especially so if boating where there is considerable current. If this is NOT the case, and water current isn't a large player in the shape of the wake, then one could argue the 0.1 precision isn't really necessary.
Secondarily, isn't precision more important than accuracy? Assuming the system is consistent, doesn't controlling speed to 0.1mph precisely become more important than if the dash reads 11mph or 14mph (aside from the annoyance of the missing decimal place on a $80k boat)? I think that is what I'm reading, but want to make sure I understand the issues (and maybe some others) as I read along.
With those two questions asked/answered I have a couple more.
Does the 242XE have a paddlewheel sensor? If so, then is it possible that the readings that are shown during water sports speed control are water speed, not GPS speed? Could this be the reason that the accuracy is seen as incorrect when compared to another GPS based speedometer?