Hi,
I did a couple of experiments. I wondered if I could tell you?
I tried riding my bike with a 1.5L bottle of mineral water in the frame in place of the battery. That was to see what it would be like with the 1.5kg range extender battery instead of the 3.6kg 625wh battery. I do think it is quite a lot better. It's hard to carry the 3.6kg battery out to my bike in the first place, and then with the battery in the bike, it is hard to get the bike out of the bike box and down the side passage to the road. Doing all those things was much easier with a 1.5L bottle of mineral water inserted in place of the battery. When I got to the road, the difference in weight was sufficient that the bike became feasible to ride on the flat as a manual bike without needing the motor, so that was a big difference.
I then put the big battery back on and adjusted the power down from 600w to 490w using the flow app. I'm hoping that that tells me roughly what a reduction in power to 80% would show as I understand that that is what I would get with the range extender battery. It reduced the lurching significantly and when I took the bike to go up the steep hill, the power was noticeably reduced, but still fine. There is only one hill in my town and the rest is all flat, so I think the cost/benefit analysis suggests that having the lighter battery would be worthwhile.
The only tricky thing is that I will need to buy the range extender battery new for £400+ and find a way to sell on the nearly new 625wh battery. The shop can't do a swap because the battery is slightly used. I am planning to ask if they would buy it back for a used price (perhaps reduced by £50) to sell on to someone else. I will see what they say.