well, it does look like you are getting a 6-7% distance increase by using the x-step
basically, your x-step may not be getting as much out of your hips as possible.
the x-step SHOULD give a substantial increase in hip rotation/explosion, which in turn SHOULD generate more arm acceleration.
i would suggest looking at the videos of some of the bigger throwing pros to see what i am talking about.
i know for some people that have a good stationary throw, they often get less hip rotation when they use an x-step, but that is due to having a poor x-step. the directions your toes point during the x-step is critical in priming your hips and allowing for a free range of motion/rotation.
you would probably be better off building an x-step FROM your stationary throw. e.g. the way your legs interact during the cross step to pivot step should feel similar to when you throw without a runup.
btw, if you are strong-arming the throw without leading with your larger muscle groups, it would make sense that the x-step isn't adding much power.