Actually, neither the Ignitor II or III REQUIRES replacing the stock coil although they state "For maximum performance it's recommended you use..." one of their "Flamethrowers".
The "Adaptive Dwell" feature will tailor the coil charging cycle to the type of coil connected. Since the low-impedance coil will charge faster the "Adaptive Dwell" feature can maximize its output potential although, to be honest, I have my reservations that in the real world it's all THAT much better than a stock coils performance.
My previous statement that the Ignitor 2/3 both REQUIRE coils with matching resistance (so no OEM Ford coils) was from my past memory of when I researched their products for my '66. However, I just spoke with the PerTronix tech department and they confirmed what I stated:
Ignitor 1 can use a stock Ford coil but increased performance is provided by their Flamethrower 1 coil. If switching to the Flamethrower 1 coil, the pink resistance wire must be bypassed for the coil.
Ignitor 2/3 can NOT use an OEM Ford coil as it will cause the Ignitor module to burn out. Maybe people have gotten away with using the Ford coil with Ignitor 2/3 modules but it's not recommended and the warranty will be void. Unfortunately, Pertronix doesn't have this info clearly detailed on their website.
In all instances, the Ignitor module still requires a full 12-volts, no matter what coil is used. This requires either a relay kit or another 12-volt source, which means you can't use the stock wiring and connect the Ignitor to the positive coil terminal (unless you've already bypassed the pink wire).
I agree with your assessment with the Adaptive Dwell; I didn't notice any performance difference going from the Ignitor 1 to 3 besides slightly improved warm/hot start performance. However, I noticed significant performance and drivability improvements going from dual-points (where I was using new Motorcraft parts and the points had been professionally set) to the Ignitor 1.