It's good to know. Good science is about accepting the data and less about hoping for the data
I'm about to try the same thing, going to swap back in the Nissan Matic S, send the old gear oil in for analysis (to see if it was causing gearbox damage for example) as I have nearly +5k Miles on it now.
I've been able to narrow down my extra efficiency boost to tire pressure when the temperature was climbing this month on top of the boost I got from my gear oil change. The temperature efficiency boost has no effect on the high speed driving, but does have an effect on the lower (under 55 mph) type of driving. Probably just rolling resistance related, I can feel it in the vehicle handling, the tires being inflated to 48 PSI when hot are real bouncy.
I've lowered the tire pressure this week to 40 PSI (in ambient 72F temperature) so that when the day gets hot, they top out at 44 PSI to try to eliminate that low speed boost I'm seeing. It's a slow process keeping track of all this data and I wish I had a way to monitor the gear oil temperature real-time or LeafSpy like, but don't have the time or resources to drill a temperature probe into the gear box. Of course my conclusions are not finished yet, but if I had to take an educated guess based on the data I have, I would say using the ULV gear oil seems to negate the mileage loss using the AC/Heat during a trip. The effect seems to show itself more when the temperature is colder and then slowly decrease as the temperature gets warmer. I suspect it is something more thermal related in the system as a whole and the benefit of the ULV is more about it's lower viscosity at colder temperatures than how well it keeps the gears lubricated. It could also be that just changing out the gear oil period is the key, regardless of what type of you choose, you will see a benefit. Maybe it's not so much a gain of efficiency but a return to the "original" efficiency when the vehicle was brand new from the factory. I think I learn more towards that than anything, but I don't have a brand new 2021 Leaf with 0 miles on the odometer to test with, unless someone wants to send me one?