Hey Rob,
I 've heard that the HPI is not the most reliable ignition and that you have to use resistor spark plug types , with resistor spark plug caps.
Our Accent ignition with 3 Ohm Dyna coils and a planemodel battery works great.
And we don't use resistor spark plugs , we use ND irridium racing spark plugs, expensive but a spark like a lightning.
I removed the resistors from the sparkplug caps and replaced them with iron pins. No resistance anymore.