The only mag I ever saw that had been running with these booster things in the plug leads had HT pick-ups that looked like pumice stone - literally - from arcing on the inside, and the HT winding (not long rewound) gave up soon after. That was a K2F on an 88 Dommie. Shouldn't need them, and like MG, best spend the money in the nearest beer tent.
As for ethanol, apart from having bad consequences for fuel lines made of old materials, I haven't noticed anything different in plug terms. French jungle juice is allegedly well 'ethanolised' but the plugs look just the same on my bikes (a bit black) as they have since unleaded came in. There's no benefit, generally speaking, in running on octane ratings higher than the engine needs NOT to pink or otherwise misbehave; but maybe there are a few more calories in 103 Avgas, a bit like in a chocolate éclair or a very good ale.
Broadly speaking, I'd say that any mildly-tuned engine that needs add-ons in any department to make it go has something on the cusp of failure or set up wrong. Resistor plug caps, like 'butterfly' Champions, seem to me to create no difficulties with mags, and the modern NGK jobs ditto.
