I have had strange problems with HEI modules. Strangest being setting initial timing at 20 degrees while running an engine on the dyno and the engine started running bad so I check the timing and its retarded 20 degrees. Replaced module and all was good.
Testing these HEI modules on an oscilloscope you can see the power start droping off at 4200 rpm. With a good aftermarket coil you do get a more powerful spark but the power does still drop off at 4200 rpm and by 6200 rpm it really gets too random to get a good spark.
The cure is a good ignition box like MSD. Then you get full spark power to 6500-7000 rpm no problem.