The stock ignition coil packs that come standard on 2014-2019 Ford Fiesta ST's work (maybe) perfectly fine (sometimes), as long as you remain stock... but what is the fun in that?
To correctly understand the benefits, we need to understand how an ignition coil works.
The way a coil works is by inducing voltage (12v) and shutting off the voltage. That time in between is called dwell time. Imagine a kitchen faucet turning on and off, and the voltage is the water. Once the voltage is "turned off" the magnetic field collapses. There are multiple windings within the coil itself: the primary winding increases the voltage to around 200v then the secondary winding multiplies that voltage to roughly 20,000v. This ends up creating a very high voltage spark. The difference between the Focus ST (FoST) coil and Fiesta ST coil is that the FoST coil actually has double the amount of windings. These windings are what causes the greater spark energy.
The FoST coils can be dropped in the FiST without tuning, but the dwell time needs to be adjusted to fully take advantage of the extra coils. Most modern ignition coils tend to saturate around 4ms of dwell time at 14v that is the point at which the coil is fully “charged” to fire. Over dwelling causes excessive heat, under dwelling causes poor spark. The OEM dwell data is close enough between the Focus and Fiesta to not cause any issues as far as running because the output voltage is still higher than a factory coil. The higher the output voltage the wider the plug gap can be. With a wider gap, more air and fuel can be packed into the cylinder and the spark still jump the gap.
Now let's talk about fuel... On high ethanol contents at high mass flow, more fuel molecules are packed when compared to pump gas. (E85 takes more fuel volume to make the same power...) This means there are more fuel molecules packed in at the same exact horsepower/massflow as on pump gas. When everything is packed tighter together, the spark is needed to jump the gap. If not it “blows-out” just as a candle would get blown out by a gust of wind.
With stock coils sometimes that candle (the spark) won't blow out and sometimes it will, so you can tighten the gap and that seems to work just ok. Or.... you can introduce a higher voltage which in turn gives a stronger spark (the FoST coils) This means you can make 430whp on full e85 at 0.032 plug gap with the FoST coils and with factory coils your gap would need to be 0.016. With the FoST coils, there is less of a chance of that spark getting blown out.
To summarize, spark plug life is still greatly improved (especially if you are tuned) on 91 or 93 but when running higher ethanol content, it is imperative to have stronger spark to protect the integrity of the combustion cycle.