I think these discussions with out specifics of the coil and ballast values are not very useful. Also, the health of the motor (wires, plugs, compression, mixture quality). As RPM increases the actual charging time available reduces quickly before the next firing. By the time you're at 6000 rpm, there is only 2.5 msec of charge time between firings (even less as there is a plug spark duration too), and coil stored energy is significantly less. It is no longer about the box, but everything else.

Yes there are box differences that matter, but just because several different folks use the same box, doesn't mean they will all see the same results in my opinion.