The biggest issue with IGHs is the low end input torque. A smaller chainring running to a 25t rear cog can exceed the input toque for the hub and tear loose some teeth or pawls.
IDK about S/A hubs, but for example, the Rohloff and Alfine recommend keeping input torque ratio Above 2:1… that is, for a 25t rear cog, a chainring no smaller than 50t. and that is their recommendation for just RIDER… not factoring in the additional torque of rider plus motor.

The nice thing is that you can generally go as small as you like on the rear cog, which is where you will get your high gear extended, because that actually lowers input torque.
The difference between the low gear with a 20 versus 25 will be minor in terms of feel…( especially considering that its in the low gears where the motor is the most help ) but it becomes very noticeable at the higher gear ratios.

The S/A 8 has a 300% Plus gear spread…. so I think you will be much happier than with a 3 speed.

I swapped out a 23t cog on my Alfine for an 18t… the bike still climbs a 20% grade just as well…. but it added nearly 10 mph to the top speed in highest gear.