Hello John,
I started messing around with these ignitions about a year ago, and partnered with Larry McAfee to evaluate them. Larry's son Kevin is actively racing C,D&E Mod in NBRA and did very well running these ignitions last year.
The distributor is universally used across the Merc650 (4-cyl) to Merc850 through the late 70's. They are battery operated but triggered by the distributor. The large coil you spoke about was used on the 650's+ but the 85hp version came out with the smaller coil you see on the Looper. The coils (both) are bullet proof and generally never fail.
Larry built a tester that drives the distributor with a air driven die grinder (in excess of 11,000rpm) and a pressurized chamber with a plexiglass top that houses the spark plugs. With a simple wiring harness and battery the whole system can be bench tested at RPM and under simulated cylinder pressure. The cylinder pressure is critical, as it effects the spark in a big way. We can see no difference in the large vs small coils. Thus the small coil is able to charge/discharge (saturate) as well as the larger version. The switch box and coil can be mounted in the boat or on the motor - your choice. The switch box does draw in excess of 5 amps (less than 7) at higher RPM, so don't skimp on the wire size.
The CDI/Rapair replacement unit is a bit more efficient (uses less power) than the Merc unit. It requires the use of their coil with their switch box. They also make a replacement trigger which is housed in the base of the distributor. Their trigger uses magnetic switching vs the optical switching of the Merc unit. Magnetic is preferred over optical. Thus they provide a steel disc for the trigger. BTW, there is an up/down orientation on the disc that must be followed for correct timing. They are usually labeled.
That's a crash course in the Type IV ignitions. Good question, hope this helps to answer your question. I should warn you that some of the switchboxes have a rev limiter installed. I usually buy the 6cyl version to avoid that - Roger