So youre saying that using the ign terminal as a 12v source for the coil and pertronix will starve it for current at some point?
Only way to tell for sure is to run your wire from the ignition terminal out to the Pertronix coil and distributor module and then while the engine is running (ignition system functional and putting an operational load on the ignition terminal you ran the wire to) measure the voltage at the + side of the coil, both at an idle and at rpm's. If the voltage reading is any less than the 12.5 to 13.5 volts then yes, you are starving it of the power it needs to function properly and live a long and happy life. Pertronix sells an ignition relay kit for this very reason, but you can put your own together for less money withh off the shelf parts from an Auto Supply store.
While the following is a slightly different example, I'll use as a real world illustration. Yesterday a good friend of mine brought his 68 Camaro over so we could re-curve his distributor and dial in his ignition timing. BTW, it's a rebuilt points distributor with brand new Pertronix Ignitor III module and Flamethrower III coil which we wired with a relay as the ignition terminal didn't supply enough voltage/current. With the timing done by lunch time he mentioned his A/C (aftermarket) was not working and never had for the last four years since he took ownership of the car.
Started troubleshooting the A/C system and found the compressor clutch would not reliably kick in and stay engaged for more than 30 to 40 seconds at a time and then it would only kick in if you tapped it into the engaged mode and then the eventually it would kick out again even though the electric clutch was receiving power. Before condemning the electric clutch as being faulty, I measure the voltage at the clutch, only 7.1 volts present which is obviously far less than it needs to function properly. It had been wired into the existing wiring harness (most likely the ignition circuit) and was far too great of a load for it.
We added a separate power feed from the horn relay bus bar with an auto reset circuit breaker and a relay using the electric clutch wire to trigger the relay and for the first time in four years his A/C worked flawlessly and while sitting there idling in 88 degree heat, it cooled the interior to 64 degrees in about ten minutes. He let me know later that evening that his new stereo system which has never worked properly is now working as it should as well. The stereo is most likely drawing it's power from the same circuit the A/C clutch was and was being starved for power as well. Again, these cars were designed for their original wiring to support only what they were built with and not much more. Starve ANY electrical component (except perhaps an incandescent bulb) of the current and voltage it was designed to operate with and the question is not if it will, fail but when it will fail.
Just look at the difference headlamp relays make in these cars, same thing is taking place. The old, undersized wiring begins to act like resistor wire and can't handle the load ending up with voltage drop, but unlike most electronics, headlamps won't fail sooner due to low voltage, they just don't produce as much light, but they will tax the rest of the headlamp wiring and the headlamp switch.