There is no such thing as a "differential" speedometer. Asking riders to obey a 15 mph speed differential is no different than asking them to just not split much faster than the cages they're passing. That's fine, and all, but any effort to codify a numerical differential is doomed to failure in the absence of a mythical differential speedometer.
For one, without such a device, no one really knows how much faster than the cages they're riding. If it becomes a "law" that one cannot exceed a 15 mph differential, there's no rigorous means of complying. And who wants to befuddle their riding brain with mental math calculations while splitting? What, are they supposed to crane their necks to see the speed readout on the cage dashboards, and compare to the reading on their moto?