The aviation spark plug gap should always be what the MFR recommends ~ 0.015" to 0.017" or what ever the latest tech data states.
Do note the very small gaps traditionally specified for aviation plugs assume magneto ignition. It is a compromise, and a nice illustration of "smallest initial gap which will get the job done".
The practical problem with magnetos is that primary voltage is dependent on the angular velocity of the rotor, i.e RPM. At low RPM, primary voltage is low,
thus secondary (plug) voltage is low. It's the reason for the impulse coupler, a device which artificially boosts RPM at the right moment.
Anyway, with our av engines a small gap can be ionized with small voltage, while still providing enough spark exposure to light friendly mixtures at normal RPM. Less friendly mixtures (more rich or more lean) are lit more reliably with wider gaps and more duration. A mag will fire a wider gap at cruise RPM, but the engine may be hard to start, a problem which will worsen with plug wear and conductive deposits.
To the OP's question, can you run wider gaps with a aviation plug? Yes, in the context that the
plug doesn't care. The typical Slick mag may not get'er done at cranking speed. An EI, however, has an external primary voltage supply, so it produces full secondary voltage at any speed which allows its trigger to operate.
So, you ask, why not just use a wide gap with this EI? Remember, secondary voltage rises until the gap ionizes, then falls as the coil's stored energy is discharged. If the gap is too wide, the secondary voltage will continue to rise until it pushes the electrons through some other discharge path, which may be a weak spot in the coil or spark plug wire insulation. It is why almost every ignition manufacturer states "Do not fire the ignition without grounding the plug leads."
I know nothing about the coil used in the Surefly. It may or may not have robust insulation properties. What I do know is an old Slick magneto harness is not going to have super duper high voltage insulation, not like some of the spark plug leads available for automotive ignitions. A gap which is no wider than actually needed is simply easy on everything.
Illustrations from the
Bosch Automotive Electric/Electronic Systems Handbook:
.