Even with a 40 kWh pack, the 6.6 kW OBC is fine. I agree with @Nubo, bumping that up, even to 7.7 kW, isn't very significant...
Assuming no losses, transferring 38 kWh of energy into the pack would take roughly 5 hrs @ 7.7 kW. At 6.6 kW, it would take about another 45 mins. That's assuming going from VLBW to 100% SOC, which should be rare.
For battery longevity, I don't think I would go down to LBW on a 2018 very often, as it would be infrequently that I would need the full 150 mile range of the 2018 in a single day.
EV batteries definitely last longer if the DoD is less than 80%, less than 60% is even better. As a current owner of a 24 kWh Leaf, I don't have that same luxury (keeping DoD low), so I don't expect my Leaf's pack to last as long as a 40 kWh pack will in a 2018. It's another reason why Tesla packs show less degredation, because the DoD is much lower for a typical daily commute.
In other words, if one's commute requires 15 kWh of energy on average, that's more than 70% DoD on a 2013 SV. On a 2018 Leaf, that would represent a DoD of less than 40%. On a Tesla with a 90 kWh pack, that same commute would represent less than 17% DoD.