Yeah, I had a LEAF, too. I also monitored battery temps on it - they were not excessive by any means and very similar to the temperatures of the Model 3. Except with the Model 3 it's easy to drive all day and you can see battery temps stay quite high in comparison.My degradation experience is as follows
2013 Leaf, passive air cooling, 40% lost at 40k miles.
2015 Soul EV, active air cooling, 15% lost at 45k miles.
2018 Model 3, active liquid cooling, 7% lost at 70k miles.
Interpret as you wish.
You can put the Model 3 batteries into a LEAF w/out any cooling, same as the LEAF and you'll still have significantly less rates of capacity loss. The batteries themselves are that much better. Will active cooling help? Absolutely - but when your average temperature is close to the passive cooling temperature target, you're not going so see substantial differences in rates of capacity loss.
The Model 3 NEEDS active liquid cooling to support the high discharge and charge rates. The LEAF got away without it because of the low power / charge limits and limited range which naturally limited how much driving you could do in a day.