The information that I have on the ****** Dart pretty much agrees with what you said, except that people who have actual examples of them confirm that the RX-1 Thruster was indeed an F100-0 booster motor. In the meager literature on it that is available online, FSI stops short of stating that the Dart-RX-1 Thruster combination will go supersonic, only that it "has supersonic potential." In the kit plans there is a general description of transonic and supersonic flight, and the claim that the Dart had exceeded the speed of sound in flight tests. They reported that the sonic booms sounded like the report from a small caliber rifle, and that they were only audible where the edges of the cone of sound that radiates from the tip of the nose cone intersect the ground. They also said that the supersonic portion of the flight was only momentary at an altitude of between 1200 and 1500 feet. Finally, they add that because the exact speed of sound depends on such factors as the elevation above sea level, the local air pressure and air temperature and other factors, the ****** Dart "cannot be guaranteed to exceed the speed of sound under all conditions." This should all be obvious, anyway. From what I have read, it is not all that easy to get even a high power rocket to go supersonic.
BTW, I just finished building a clone of the Dart last week. It has an 18mm mount in the sustainer. I have created the needed parts to construct a faux RX-1 Thruster (that will actually be a reusable booster stage powered by D12-0 motors), but I haven't assembled it yet. Obviously my clone won't "have supersonic potential" but, in accordance with the kit instructions, I left a launch lug off of the sustainer. If I want to fly it in single stage configuration, I will need to launch it from a tower or a piston, or use a fly-away lug. When Quest releases its 28mm black powder F motor, I will probably be able to use it as a modern analog of the RX-1 Thruster. It may even be more powerful than FSI's motor was.
In EMRR's RockSim Library there is an archive containing motor files for most of FSI's motors, including the never-released G60. The library also contains a RockSim file of the ****** Dart/RX-1 Thruster combination that was created by Steve Naquin. A sim of a flight (from my location) on a F100-0/D20-7 motor combination yields a maximum velocity of 1098.95 fps and a maximum altitude of 3401 feet. That max velocity just reaches into the supersonic range, I believe. But of course, this is only a simulation.
MarkII