Total coil circuit resistance with the stock ECU is designed to be 3-4 ohms, when hot. This can be in the coil plus the ballast, or it can be in a higher resistance coil alone.
For stock coil + ballast, coil resistance is around 1.5 ohms and ballast is 0.6 ohms cold and around 2 ohms hot. (It takes about 30-60 seconds for the ballast to heat up to near final hot resistance.)
A 3 ohm 'no ballast' coil comes pretty close so seems to work OK. The hotter spark produced when starting cold (due to the low cold ballast resistance) is not present with a fixed 3 ohm coil. Using the ballast in series with a 3 ohm coil would reduce the coil current and thus spark energy, cold and hot, so you'll just end up with a weak spark if you try that.
