I know several brands now offer.
Riese Müller have (with two battery pack but total is kwh)
Pedego as i heard offering 30ah one
Ariel Rider has 25ah battery
So 17.4 or 21ah seem ok for me.
Dunbar about your last mentioned point i have to add something. You are totally right about continuous power and current need. But those Panasonic or Samsung e-vehicle (for the ones who mentioned they are e-bike battery cells are wrong. They are e-vehicle cells) are designed for peak consumption.
The battery cell can play an important factor in "peak " cases. If you start to climb uphill or need a sudden acceleration. In those cases choice of cells can play a difference.
For juiced i am not sure if this is the case. I still have a feeling of a cheap bike about it. Maybe i am right maybe i am wrong. I will give a try about it as it is even compared to "Stromer" so let's check and compare if it is really comparable to Stromer.
If it is so, then i will buy one or maybe two (one for fiance ) . As their kickstarter price is really unbeatable.